API: Include ApiMain construction in api.php try-catch block

If the User language is accessed from ApiMain::__construct on a request
via OAuth and the authorization headers weren't valid, the
UsageException was not being handled.

Further, the code setting $wgLang in that constructor should be using
the language just set on the local context, not the main request
context.

Bug: 72384
Change-Id: I90900dc4a8aea22c67c0c05b38128d5dbb044f07