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
I have changed it into HintException, but could not understand the use of Hint argument.