Don't crash on pathwatcher failures

Instead, increase polling interval for that file/dir to 500ms from 5s.

Fixes #3336.

The most common case where this was occuring is when you're on Linux and
your inotify max_user_watches is too low.  A wiki page will explain how
to increase this.  In debug mode, a message will tell you to go to the
wiki page if you hit the limit (we may later expose this message by
default, but it is part of the new user experience).