jqueue: Improve inotify error reporting

This addresses issue 218. When the number of inotify watches is
exhausted, for example by being set too low from the beginning or by
other programs, waiting for a job to change would just report a lost job
(e.g. “Error checking job status: Job with id 7817 lost”).

This patch changes the job watcher to no longer catch
“errors.InotifyError” and, this is by far the larger part of this patch,
adds unittests for this situation.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>