[r=rogpeppe] state: avoid panic in watchers
If the state is closed underneath a watcher, the
underlying watcher.Watcher will die with no error.
That should not itself trigger a panic.
I have not added test coverage for every single
entry point that creates a given kind of watcher.
I feel that adds unnecessarily to the test burden, and
with this particular kind of test, inspecting the
test coverage shows clearly which watchers are
tested in this way.
https://codereview.appspot.com/13386044/