CachePlugin: Continue when file is deleted

With the CachePlugin, if a file is deleted it displays this error:

```
>> Error: ENOENT, stat '/Users/Kyle/Documents/www/ynab/ynab_web/src/components/deleteme.coffee'
```

After that, subsequent compilations fail requiring you to restart the watch.

This PR will ignore any files that don't exist in the cache and allow subsequent compilations to run.