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