[watchman] fix log spew about expected exits
[watchman] fix log spew about expected exits

Summary:
when watchman restarts or gets a fatal error, we raise certain exceptions to intentionally exit `WatchmanListenLoop`.

before, we were logging an "unhandled" exception, so here I am handling the ones we expect.

Reviewed By: nmote

Differential Revision: D27621169

fbshipit-source-id: 966d1d012736645281c73fd14c5d955f21724cc5

mroch/flow,facebook/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow