On windows, the file path contains back slashes

Ensure that resourceURL is transformed to replace back slashes with
forward slashes
Handle and emit errors from Watcher

This prevents the Flo session from crashing, making it more resilient,
and gives consumers of Flo the opportunity to handle watcher's errors.
Merge branch 'master' of https://github.com/facebook/fb-flo
Remove my original fix for windows paths

Given that it has been properly refactored
Merge pull request #47 from tyson-benson/master

File watcher throws errors sometimes, now Flo handles them