cancel root if exception bubbles up in io/notify threads
cancel root if exception bubbles up in io/notify threads

Summary: Log the exception message and cancel the root

Reviewed By: farnz

Differential Revision: D4201423

fbshipit-source-id: d10a81170d4b21faaab5b62a25bf5e7d64740c65

wez/watchman,kwlzn/watchman,facebook/watchman,kwlzn/watchman,wez/watchman,nodakai/watchman,wez/watchman,wez/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,wez/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,wez/watchman,wez/watchman,kwlzn/watchman,facebook/watchman,facebook/watchman,kwlzn/watchman,kwlzn/watchman,facebook/watchman,kwlzn/watchman,kwlzn/watchman,wez/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,wez/watchman