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