[Enhancement] Unify the JSON response bodies of the register, login and forgot password actions when performing AJAX requests

[wblommaert]

A colleague of mine was busy implementing validation on AJAX forms for the registration, login and forgot password forms when he noticed that the aforementioned all return a different JSON structure. The structures as they are right now are:

Register:
{"errors":{"username":["Username \"email@domain.com\" is already taken."],"email":["Email \"email@domain.com\" is already taken."]}}

Login:
{"errorCode":2,"error":"Invalid email address or password"}

Forgot password:
{"error":["Username or email address invalid."]}

It could be interesting to unify these response bodies into something more uniform that allows easier validation whenever these forms are approached via AJAX.

[brandonkelly]

We should especially consider this when building out a REST API (#815).