Define errorMessage as an informal protocol for returning error message representations rather than the description method. Document RKErrorMessage.

Code/ObjectMapping/RKErrorMessage.h

@@ -21,13 +21,28 @@
 #import <Foundation/Foundation.h>
 
 /**
- A destination class for mapping simple remote error messages.
+ The `RKErrorMessage` is a simple class used for representing error messages returned by a remote backend system with which the client application is communicating. Error messages are typically returned in a response body in the Client Error class (status code 4xx range).
+
+ ## Error Message Informal Protocol
+
+ The `errorMessage` property method is the sole method of an informal protocol that must be adopted by objects wishing to represent error messages within RestKit. This protocol is by the `RKErrorFromMappingResult` function when constructing `NSError` messages from a mapped response body.
+
+ @see `RKErrorFromMappingResult`
  */
 @interface RKErrorMessage : NSObject
 
+///-----------------------------------
+/// @name Accessing Errror Information
+///-----------------------------------
+
 /**
- The error message string mapped from the response payload
+ The error message to be presented to the user.
  */
-@property (nonatomic, strong) NSString *errorMessage;
+@property (nonatomic, copy) NSString *errorMessage;
+
+/**
+ A dictionary of application specific information that accompanies the error message.
+ */
+@property (nonatomic, copy) NSDictionary *userInfo;
 
 @end