proxy-only KeyError when file fails to download

[redshiftzero]

If there's an error during downloading a submission, the key filename will not be present in the response body (see relevant code here): instead the "error" key will be present.

Thus if a download error occurs, we'll see a KeyError: 'filename' from the SDK due to this line:

filepath = os.path.join(
                "/home/user/QubesIncoming/", self.proxy_vm_name, data["filename"]
            )

We should fix this once the exception handling in #110 is merged (related).

[redshiftzero]

Just checked, this was fixed by #110, added a regression test in #112