[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
But this is what it would do:
First iteration:
`$result = true && unlink()`
If unlink failed, then `$result` becomes false.

Second iteration:
`$result = false && unlink()`.
It will not try to run unlink() from here because `$result` will stay false.

Either there is something wrong with this logic which doesn't seem to do what you said, or I didn't understand it properly.