Add a warning and fallback on polling mode if limit exceeded
Add a warning and fallback on polling mode if limit exceeded

Works only on linux (not sure how error is named on other systems):
```
*** System notification limit is too small, falling back to polling
mode.
*** For better performance increase system limit:
   sysctl fs.inotify.max_user_watches=524288
*** Polling for changes every 1000 ms
```

Fixes #62

mattgreen/watchexec,mattgreen/watchexec,mattgreen/watchexec,mattgreen/watchexec