remove catch-all from Watchman.init
remove catch-all from Watchman.init

Summary:
`Watchman.init` has one caller, in `FileWatcher.WatchmanFileWatcher.watchman.wait_for_init` (see [here](https://github.com/facebook/flow/blob/f89a6676251e0bda34f32ad61a5125ec7a34c37b/src/monitor/fileWatcher.ml#L383)).

we were raising exceptions just to catch all of them except `Exit_with _` and `Watchman_restarted`. those two will then be caught by the catch-all in the `go` helper linked above and logged as unexpected exceptions.

instead, we handle them gracefully within watchman.ml because they are expected.

Reviewed By: nmote

Differential Revision: D28432575

fbshipit-source-id: f41aecdf143d415ea6ca87a2e9b3f32e8d5ba7f3

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