Fix OAuth token refresh return value

Summary:
Ref T1536. Ref T2852. Currently, after refreshing the token we don't actually return it. This means that code relying on token refresh fails once per hour (for Asana) in a sort of subtle way. Derp.

Update bin/auth refresh to make this failure more clear.

Test Plan: Set force refresh flag and verified a return value.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T1536, T2852

Differential Revision: https://secure.phabricator.com/D6295