Throw an exception if any of specified app paths are not found

E.g. when we have app paths in config, but they do not exist,
currently we do not show any exception messages or write into log.

Try:

* Rename apps directory into something else
* refresh OC page.

Expected: A warning message or an entry in log

Actual: Just infinite loop warning by Chromium

This writes an exception to the log
You could also use a HintException that takes a hint as second argument.
I believe this was designed to be able to be displayed in a friendly format on the main page.