Removed error from filesystem.Watch.
Removed error from filesystem.Watch.

The only error we returned was on cancellation, which isn't very useful.
In the event of multiple poll failures, we may want to fall back to a
timer that fires every N seconds to cause a rescan, but we can implement
that later. If a root can't be polled, there are likely bigger issues.

havoc-io/mutagen,havoc-io/mutagen