Fixing mediawiki.action.watch.ajax

Fixing mediawiki.action.watch.ajax
* Adding error capturing in case there is an error (either in the API or with the request itself). Right now if it fails it just keeps reading "Watching..." and no error is displayed.
This lack became visible when r88522 changed ApiWatch to require a token and POST.
* Added message 'watcherrortext'.
* Moved messages-array in Resources.php form legacy.ajax to action.watch.ajax (looks like this was forgotten in r78147)
* Switched it to make a POST request in preparation of making it work with the changed API backend as of r88522.