R1:f0857e4fd817

Improve error message for bad timestamps

Summary: Ref T3031. While we should probably do more than this, provide a more useful error message so I don't have to make users run `date` and such.

Test Plan:
Added `|| true` and ran `arc list`:

$ arc list --conduit-uri=http://local.aphront.com:8080/
Exception
ERR-INVALID-TOKEN: The request you submitted is signed with a timestamp, but that timestamp is not within 15 m of the current time. The signed timestamp is 1375454102 (Fri, 02 Aug 2013 07:35:02…