title: RestKit 0.20.0-rc1 - How do I map multiple error parameters?
body: <p>I'm currently using the RKErrorMessage class to map an error message from my server like so:</p>
  
  <pre><code>RKObjectMapping *errorMapping = [RKObjectMapping mappingForClass:[RKErrorMessage class]];
  [errorMapping addPropertyMapping:[RKAttributeMapping attributeMappingFromKeyPath:@"message" toKeyPath:@"errorMessage"]];
  RKResponseDescriptor *errorResponseDescriptor = [RKResponseDescriptor responseDescriptorWithMapping:errorMapping
                                                                                              pathPattern:nil
                                                                                                  keyPath:@"error"
                                                                                              statusCodes:RKStatusCodeIndexSetForClass(RKStatusCodeClassClientError)];
  </code></pre>
  
  <p>But now I would like to map an additional parameter from my server, for instance the error title (or anything else for that matter). My JSON looks like this:</p>
  
  <pre><code>{
      "error":{
          "title":"This is the error title"
          "message":"This is the error message",
      }
  }
  </code></pre>
  
  <p>The idea is that I can directly map some of these error messages to UIAlertViews if something ever goes wrong when my app is accessing the server. This can be done already with <em>only</em> the error message but adding a title seems to make it fit well with UIAlertView.</p>
  
  <p>From looking at the RKErrorMessage class I see that it only has two parameters: errorMessage and userInfo. How would I go about doing this?</p>
  
  <p><strong>UPDATE:</strong> The reason I'm trying to do this with RKErrorMessage is that it automatically maps to the NSError passed into the failure block in the RestKit get/post/put/etc methods... which is pretty nice.</p>
  
  <pre><code>[objectManager getObjectsAtPath:@"/api/somethingsomething/" parameters:nil success:^(RKObjectRequestOperation *operation, RKMappingResult *mappingResult) {
  
      } failure:^(RKObjectRequestOperation *operation, NSError *error) {
  
      }];
  </code></pre>