* Remove I/O in event loop for add-on backup and restore
Remove I/O in event loop for add-on backup and restore operations. On
backup, this moves the add-on shutdown before metadata is stored in the
backup, which slightly lenghens the time the add-on is actually stopped.
However, the biggest contributor here is likely adding the image
itself if it is a local backup. However, since that is the minority of
cases, I've opted for simplicity over optimizing for this case.
* Use partial to explicitly bind arguments
* Bad message error marks system as unhealthy
* Finish adding test cases for changes
* Rename test file for uniqueness
* bad_message to oserror_bad_message
* Omit some checks and check for network mounts