[appserver-io/webserver] No error shows up if documentRoot is not available

[htuscher]

I tried changing the documentRoot to a faulty symlink.
Appserver just ignores this problem and serves the welcome page. No error log, nothing, no hint.
After I recognized my documentRoot symlink was bad it worked, but it took me more than an hour.

Please throw an error or a warning if the documentRoot is not available/readable.