Clean up the mess that is wfDie (resolves r85918). wfDie() doesn't exist anymore...

Clean up the mess that is wfDie (resolves r85918). wfDie() doesn't exist anymore, so don't use it.

The only two usages left since r91590 are php version checks for index/load/api.php, and $wgAPIEnabled check in api.php. Consolidate all of the "bailing for invalid version" into PHPVersionError.php. Leaving $wgAPIEnabled as the only user was silly, so just echo and die like wfDie() would've done