Fix Admin Checks silently failing after Exception

If an Admin check throws an exception, it is caught by the default
exception handler and MantisBT triggers an ERROR_PHP.

This in turn is caught by check_error_handler(), which just stores it in
$g_error_raised, but since code execution can't continue it is never
reported to the end-user, which causes confusion and makes debugging
of the admin checks difficult.

To avoid this, check_error_handler() now detects if it's processing an
E_USER_ERROR of type ERROR_PHP and lets PHP handle it.

Fixes 0033010