storage: raise exception in case of healthcheck not found

Somehow related to #6.
backends: fix remove_watcher behaviour

The API should remove the user only when it's member of no other group,
otherwise it should just remove the user from the group.

This is not even close from being "parallel-safe", but I don't think we
care right now.

Closes #6.
tests/backends: add tests for get_value
tests: fix flake8 issues