Degrade silently bit.ly API call on request failure

Summary:
If a too long URL is generated by the list replace tool, bit.ly API
throws a 500 error. To prevent this, we degrade silently to a regular
non-short URL mode when this occurs.

Fixes T601.

Test Plan: test with URL provided in bug case

Reviewers: dereckson

Maniphest Tasks: T601

Differential Revision: http://devcentral.nasqueron.org/D95