remove use of Not_found in watchman.ml
remove use of Not_found in watchman.ml

Summary:
2 motivations for this refactoring:

- using `result`/`option` instead of exceptions ensures failures are handled. we see a lot of unhandled exceptions out of this `Watchman` module.
- `Base` deprecates `Not_found`

Reviewed By: dsainati1

Differential Revision: D21620491

fbshipit-source-id: b7f7258be55f098b08b8a7a14985d2ef3eec4b30

nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow