Test for errors during inotify callback

Test for errors during inotify callback

- Create a new _MyErrorLoggingAsyncNotifier class which registers
  error counts, rather than logging them
- Add an additional ERR notifier to test with
- Check that no error was returned, for tests that weren't supposed to
- Add a new test case for a callback that's supposed to raise an
  exception

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>