make Watchman.call_on_instance return a result
make Watchman.call_on_instance return a result

Summary: more pushing exceptions around

Reviewed By: nmote

Differential Revision: D28518483

fbshipit-source-id: cff389b6c751f97be9eff73b2ebfe1951d139580

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