unwatch should not log a warning when it fails

[maximlt]

Reviewing Marc's docstring update work, I saw that unwatch apparently logs a warning if it fails at removing a watcher, there has to be a better way, e.g. raising a LookupError.

    def unwatch(self_, watcher):
        """Remove the given Watcher object (from `watch` or `watch_values`) from this object's list."""
        try:
            self_._register_watcher('remove', watcher, what=watcher.what)
        except Exception:
            self_.warning(f'No such watcher {str(watcher)} to remove.')

[jbednar]

I don't know of any reason it wasn't always an exception, but maybe someone else remembers.