Handle overwrite issue instead of leaving to svn…

Current error:

```
$ git grab https://github.com/abc/xing-backend/tags/0.0.14/lib/xing
svn: E155000: Destination directory exists; please remove the directory or use --force to overwrite
svn: E155000: 'xing' already exists
```

Should fail with a well-formatted failure, or silently overwrite with `-f`/`--force` flag.