Only delete existent files

When specified exludes with globs, such as `moment/locale/**` for example, the directory gets deleted first, hence following files supposed to be deleted don't exist any more, which results in the following errors:
```console
Deleting tmp/crx-2y3bcped4na/bower_components/moment/locale...OK
Deleting tmp/crx-2y3bcped4na/bower_components/moment/locale/af.js...ERROR
>> Cannot delete nonexistent file.
Deleting tmp/crx-2y3bcped4na/bower_components/moment/locale/ar-ma.js...ERROR
>> Cannot delete nonexistent file.
Deleting tmp/crx-2y3bcped4na/bower_components/moment/locale/ar-sa.js...ERROR
```

This pull request adss a check if the file exists before running the delete operation on it.