Fix silent crash if 3rdparty dir is not available

Fixes https://github.com/owncloud/core/issues/9975

Regression.
Unfortunately `OC_Template::printExceptionErrorPage` would not work without 3rdparty dir.

please please please never use exit()

as soon as this code is executed via the console or phpunit this will cause the calling process to die right away - which is really bad.

throw an exception or return - whatever