Fixed a problem with canceled backups

Was an backup canceled, a restart doesn't work. The problem was that a non existing file caused an failure. This fix handles this situation for the ssh and googledocs backend.
Added FileMissingException
Added detection for throwing FileMissingException to all backends
Updated BackendManager to handle failures with delete by checking if the file was actually deleted.
Bugfix from EddiVonDerAlm@1e83373
Merge branch 'improved_delete_handler'