Fix two minor issues with "arc download"

Summary:
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.

Test Plan:

• 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