[encryption] check if file still exists before removing encryption keys

Added some checks if a file is really gone before deleting the encryption keys and some additional output to the log file in debug mode. This PR contains tests for (hopefully) all possible scenarios in which the encryption keys can be deleted.

I hope this will fix bugs like https://github.com/owncloud/core/issues/9191
If I understand well you are checking `$result` from the previous iteration.
You could have simply aborted with `break` earlier as soon as the result is false [optional]