CI: Scan PHP Stack traces [3h]

[AronNovak]

Add the check for any PHP execution to make Travis fail when something emits a PHP stack trace or warning message. For this purpose, we might even enable PHP error log, and rely on that instead of the stderr of any PHP execution, that might be web based.
The trick is that potentially during Simpletests, it can be okay to see such things, to be checked if everything is handled via error handlers and nothing goes into the logs.