Return errors as json objects instead of strings

[robbevp]

We currently have a mapping in config/locales/en.yml to convert error messages to keys, which we then look up in web's translations. I realised that by using @object.errors.errors we can just directly return the attributes of each error.

A response with errors would then become:

[
  {"attribute" => "title", "type" => "blank", "options" => {}},
  {"attribute" => "normalized_title", "type" => "blank", "options" => {}}
]

Options can be filled with more detail if needed. (For example, if we would have length validations: `{"attribute" => "title", "type" => "too_long", "options" => { "count" => 50 }

@chvp What do you think? Wanted to check this principle first, before I update the controllers, and js-client and web.

• I've added tests relevant to my changes.