Mods to support error, message, and validation errors (#260).

src/main/java/org/gitlab4j/api/GitLabApiException.java

 package org.gitlab4j.api;
 
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.Iterator;
+import java.util.List;
+import java.util.Map;
+import java.util.Map.Entry;
+
+import javax.ws.rs.core.MediaType;
 import javax.ws.rs.core.Response;
 import javax.ws.rs.core.Response.StatusType;
 
-import org.gitlab4j.api.models.ErrorMessage;
+import org.gitlab4j.api.utils.JacksonJson;
+
+import com.fasterxml.jackson.databind.JsonNode;
 
 /**
  * This is the exception that will be thrown if any exception occurs while communicating
@@ -15,6 +25,7 @@ public class GitLabApiException extends Exception {
     private StatusType statusInfo;
     private int httpStatus;
     private String message;
+    private Map<String, List<String>> validationErrors;
     
     /**
      * Create a GitLabApiException instance with the specified message.
@@ -28,7 +39,7 @@ public class GitLabApiException extends Exception {
 
     /**
      * Create a GitLabApiException instance based on the ClientResponse.
-     * 
+     *
      * @param response the JAX-RS response that caused the exception
      */
     public GitLabApiException(Response response) {
@@ -38,10 +49,50 @@ public class GitLabApiException extends Exception {
         httpStatus = response.getStatus();
 
         if (response.hasEntity()) {
+
             try {
 
-                ErrorMessage errorMessage = response.readEntity(ErrorMessage.class);
-                message = errorMessage.getMessage();
+                String message = response.readEntity(String.class);
+                this.message = message;
+
+                // Determine what is in the content of the response and process it accordingly
+                MediaType mediaType = response.getMediaType();
+                if (mediaType != null && "json".equals(mediaType.getSubtype())) {
+
+                    JsonNode json = JacksonJson.toJsonNode(message);
+
+                    // First see if it is a "message", if so it is either a simple message,
+                    // or a Map<String, List<String>> of validation errors
+                    JsonNode jsonMessage = json.get("message");
+                    if (jsonMessage != null) {
+
+                        // If the node is an object, then it is validation errors
+                        if (jsonMessage.isObject()) {
+
+                            validationErrors = new HashMap<>();
+                            Iterator<Entry<String, JsonNode>> fields = jsonMessage.fields();
+                            while(fields.hasNext()) {
+
+                                Entry<String, JsonNode> field = fields.next();
+                                List<String> values = new ArrayList<>();
+                                validationErrors.put(field.getKey(), values);
+                                for (JsonNode value : field.getValue()) {
+                                    values.add(value.asText());
+                                }
+                            }
+
+                        } else {
+                            this.message = jsonMessage.asText();
+                        }
+
+                    } else {
+
+                        JsonNode jsonError = json.get("error");
+                        if (jsonError != null) {
+                            this.message = jsonError.asText();
+                        }
+                    }
+                }
 
             } catch (Exception ignore) {
             }
@@ -50,7 +101,7 @@ public class GitLabApiException extends Exception {
 
     /**
      * Create a GitLabApiException instance based on the exception.
-     * 
+     *
      * @param e the Exception to wrap
      */
     public GitLabApiException(Exception e) {
@@ -60,7 +111,7 @@ public class GitLabApiException extends Exception {
 
     /**
      * Get the message associated with the exception.
-     * 
+     *
      * @return the message associated with the exception
      */
     @Override
@@ -71,7 +122,7 @@ public class GitLabApiException extends Exception {
     /**
      * Returns the HTTP status reason message, returns null if the
      * causing error was not an HTTP related exception.
-     * 
+     *
      * @return the HTTP status reason message
      */
     public final String getReason() {
@@ -81,10 +132,32 @@ public class GitLabApiException extends Exception {
     /**
      * Returns the HTTP status code that was the cause of the exception. returns 0 if the
      * causing error was not an HTTP related exception.
-     * 
+     *
      * @return the HTTP status code, returns 0 if the causing error was not an HTTP related exception
      */
     public final int getHttpStatus() {
         return (httpStatus);
     }
+
+    /**
+     * Returns true if this GitLabApiException was caused by validation errors on the GitLab server,
+     * otherwise returns false.
+     *
+     * @return true if this GitLabApiException was caused by validation errors on the GitLab server,
+     * otherwise returns false
+     */
+    public boolean hasValidationErrors() {
+        return (validationErrors != null);
+    }
+
+    /**
+     * Returns a Map<String, List<String>> instance containing validation errors if this GitLabApiException 
+     * was caused by validation errors on the GitLab server, otherwise returns null.
+     *
+     * @return a Map<String, List<String>> instance containing validation errors if this GitLabApiException 
+     * was caused by validation errors on the GitLab server, otherwise returns null
+     */
+    public Map<String, List<String>> getValidationErrors() {
+        return (validationErrors);
+    }
 }

src/main/java/org/gitlab4j/api/utils/JacksonJson.java

@@ -243,4 +243,15 @@ public class JacksonJson extends JacksonJaxbJsonProvider implements ContextResol
     public static <T> String toJsonString(final T object) {
         return (JacksonJsonSingletonHelper.JACKSON_JSON.marshal(object));
     }
+
+    /**
+     * Parse the provided String into a JsonNode instance.
+     *
+     * @param jsonString a String containing JSON to parse
+     * @return a JsonNode with the String parsed into a JSON tree
+     * @throws IOException if any IO error occurs
+     */
+    public static JsonNode toJsonNode(String json) throws IOException {
+        return (JacksonJsonSingletonHelper.JACKSON_JSON.objectMapper.readTree(json));
+    }
 }