MustErr on API server is unsafe

[Bodie Solomon]

Per @fwereade [1] MustErr is not safe and should never be used outside of client code.

...

   StringsWatcherId: u.resources.Register(watch),
   Changes: changes,
  }, nil
 }
 return nothing, watcher.MustErr(watch)
}

1. https://github.com/juju/juju/pull/141#discussion_r14180067

[Bodie Solomon (binary132)]

Edited title to be more correct: MustErr on apiserver is the issue.