Fix exception in 'get --continue' if files exist

If I run:

```
s3cmd get s3://mybucket/myfile.mp4
```

Then

```
s3cmd get --continue s3://mybucket/myfile.mp4
```

s3cmd throws an UnboundLocalError. Faking "response" in the catch block fixes this error.

I'm new to python so alternative ways to fix this are welcome. Thanks.