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.

In general i agree. But in this special case i´m not sure. If the 3rdparty directory is not there than all kind of other code that might come afterwards will fail and produce all kind of strange error messages. The purpose of this change is to have a nice and descriptive error instead.