Add details to error response for POST /api/v1/accounts in REST API

[Gargron]

Example error response:

{
  "error": "Validation failed: Password can't be blank, Username must contain only letters, numbers and underscores, Agreement must be accepted",
  "details": {
    "password": [
      {
        "error": "ERR_BLANK",
        "description": "can't be blank"
      }
    ],
    "username": [
      {
        "error": "ERR_INVALID",
        "description": "must contain only letters, numbers and underscores"
      }
    ],
    "agreement": [
      {
        "error": "ERR_ACCEPTED",
        "description": "must be accepted"
      }
    ]
  }
}

Note: API responses always use en locale.

This PR requires some i18n changes because the way we've been handling error messages so far is not 100% compatible with being able to pass error names from validations.

Error
ERR_BLOCKED	When e-mail provider is not allowed
ERR_UNREACHABLE	When e-mail address does not resolve to any IP via DNS (MX, A, AAAA)
ERR_TAKEN	When username or e-mail are already taken
ERR_RESERVED	When a username is reserved, e.g. "webmaster" or "admin"
ERR_ACCEPTED	When agreement has not been accepted
ERR_BLANK	When a required attribute is blank
ERR_INVALID	When an attribute is malformed, e.g. wrong characters or invalid e-mail address
ERR_TOO_LONG	When an attribute is over the character limit
ERR_TOO_SHORT	When an attribute is under the character requirement
ERR_INCLUSION	When an attribute is not one of the allowed values, e.g. unsupported locale

[nightpool]

Beyond the discussions about i18n we had in chat:

• ERR_ACCEPTED and ERR_BLANK could maybe be combined into a more general ERR_REQUIRED? or we can replace ERR_ACCEPTED with ERR_BLANK

[nightpool]

the aliases hash might be a little clearer here as a keyword argument, i.e.

render json: ValidationErrorFormatter.new(e, aliaes: {
    :'account.username' => :username,
    :'invite_request.text' => :reason
  }).as_json,
  status: :unprocessable_entity