Extend response with extra information in case of error

[alevshunov]

When we are saving/updating an entity via agrest using POST/PUT methods and the model is not valid by some rules, the response is looks like: "validation failed, 3 errors". But we have no any extra information about what kind of errors are.

So, it will be nice to have extended error response, something like:

{ 
   success: false, 
   message: "...: validation failed, 3 errors", 
   errors: [
     { field: 'firstName', message: 'firstName is re'}, 
     ... 
   ],
   ...
}