Stop the debugging view for typeahead datasources from fataling

Summary: Fixes T13119. Ref T13120. This isn't the world's most elegant patch, but restores the debugging version of this view to service.

Test Plan: Viewed debugging phage (at /typeahead/class/). Used the actual proxy (by changing a datasource custom field from the comment area).

Maniphest Tasks: T13120, T13119

Differential Revision: https://secure.phabricator.com/D19304