[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
just because I can't delete one key I don't want ti stop but delete as much as possible