[5.5] Unify the exception response format when converting to JSON

[themsaid]

Currently any normal Exception is converted to this JSON structure:

{"message": "..."}

In case debug mode is on, several other attributes appear (file, line, & trace).

However there are two exceptions to this structure, first a validation exceptions returns the following:

{
    "test": [
      "The test field is required."
    ]
  }

And an AuthenticationException returns:

{"error" : "Unauthenticated."}

In this PR I'm attempting to unify the error response by returning the following from a validation exceptions:

{
  "message": "The given data failed to pass validation.",
  "errors": {
    "test": [
      "The test field is required."
    ]
  }
}

If that PR is approved I'm going to change the response format in https://github.com/laravel/laravel/blob/develop/app/Exceptions/Handler.php#L55 as well, this will make it standard across the framework to have a message attribute in a JSON error response that consumers can use to render a message to the end user or log it somewhere for later.