_include.php: Improve handling of unhandled errors and exceptions.
_include.php: Improve handling of unhandled errors and exceptions.

This patch adds an exception handler which will catch all unhandled
exceptions and display an error page to the user. It also adds a
handler for PHP errors and warnings. This handler will show a stacktrace
of the error, and then pass the error to the normal PHP error handler.

rchavik/simplesamlphp,rchavik/simplesamlphp,rchavik/simplesamlphp,rchavik/simplesamlphp