Fix two minor issues with "arc download"
Ref T12651. Ran into these during D17799:
- Use getStatusCode() to put the actual status code into the message.
- If we fail but wrote an empty file to reserve the filename, clean it up.
- Faked the error, phlog()'d the exception.
- Saw sensible exception message.
- Saw empty file get cleaned up.
Reviewers: chad, amckinley
Reviewed By: chad
Maniphest Tasks: T12651
Differential Revision: https://secure.phabricator.com/D17800