GUI should throw errors if directory includes failed, VFS::DirectoryExists

[elexis]

If a GUI page file wants to include a JS file that doesn't exist, it throws an error:

For instance:

<script file="nope/doesntexist.js"/>

results in

ERROR: File 'nope/doesntexist.js' does not exist

But for directories it silently continues and just doesn't load any files if there was a typo in the directory name:

<script directory="doesnt/exist/either/"/>

Specifically mentioned includes should never be optional and hence always throw an error.

On first sight it looks like we are missing a DirectoryExists test that we don't have for the VFS.

refs ​Phab:D619