Validate PHP error logging

See https://gitlab.com/hres/aegir/issues/44

Where @colan said:

By default, on Apache, PHP errors get written to /var/log/apache2/error.log. However, it looks like the default PHP-FPM config (used with Nginx) doesn't write out logs like that. Something simple, like leaving off a semi-colon at the end of a line of PHP code, can result in a White Screen Of Death (WSOD), with no indication of what's amiss. Proper system logging is critical for prompt resolution of such issues.