API: Catch Errors as well as Exceptions

API: Catch Errors as well as Exceptions

ApiMain (and also api.php) tries to catch any Exception so as to provide
a properly-formatted error message to the client instead of an HTML
error page.

With PHP 7.0, some cases that produce an Exception in HHVM instead
produce an Error. The API code should catch these too.

Fortunately neither Zend PHP nor HHVM care if you try to catch a class
that doesn't exist, so we can just add catch blocks for Throwable and
not worry about it.

Bug: T202416
Change-Id: I189eee466bd09870bc172f2420be393a7c0b1900