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 -0700), and the current server time is 1375454102 (Fri, 02 Aug 2013 07:35:02 -0700). This is a differnce of 0 seconds, but the timestamps must differ from the server time by no more than 900 seconds. Your client or server clock may not be set correctly.
  (Run with --trace for a full exception trace.)

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T3031

Differential Revision: https://secure.phabricator.com/D6653