Use TextFormatter in the REST API

Use TextFormatter in the REST API

* Add ResponseFactory::createLocalizedHttpError(), which generates a
  JSON response body from a MessageValue
* ResponseFactory::__construct() accepts an array of TextFormatter
  objects. For ease of testing, the array may be empty. The integrated
  ResponseFactory has a TextFormatter for English, and one for
  $wgContLang if that is different.
* Use createLocalizedHttpError() to show helpful error messages for
  errors generated by Router.

Change-Id: I897a0aee42227916c568333ab384966f1b87f599