_log_failed_submission fails with malformed exceptions
_log_failed_submission fails with malformed exceptions

This will manifest in those kind of errors:

```
        if 'exception' in data and 'stacktrace' in data['exception']['values'][0]:
            # try to reconstruct a reasonable version of the exception
            for frame in data['exception']['values'][0]['stacktrace']['frames']:
                output.append('  File "%(fn)s", line %(lineno)s, in %(func)s' % {
>                   'fn': frame['filename'],
                    'lineno': frame['lineno'],
                    'func': frame['function'],
                })
E               KeyError: 'filename'
```
getsentry/raven-python,johansteffner/raven-python,johansteffner/raven-python,getsentry/raven-python,johansteffner/raven-python,getsentry/raven-python