mediawiki.jqueryMsg: Abort test if a language request fails

mediawiki.jqueryMsg: Abort test if a language request fails

Follows-up 365b6f3.

If one of the requests fails, stop the test. This avoids one test
from leaking into the next test if it times out. And also makes
the error in question easier to handle. Most subsequent errors
tend to be cascading failures from earlier ones in this case.

It will still run other qunit tests within this module and other modules,
it only aborts the subtest for "Match PHP parser" in mediawiki.jqueryMsg.

To test:
* Change getMwLanguage() and append a bogus string to the request url,
  e.g. 'load.fail.php'.
* It will fail as expected with 'Language "en" failed to load.' and QUnit
  reports that it expected 49 more assertions, and moves on to the next test.

Change-Id: Id77bf43ef0b7a031db2fdb1d6c1bb8b2c64e689c