Use Workflow, not Request, for Differential populate/show more requests

Summary: When a JX.Request fails, there's no default error handling. Rather than
write some kind of custom stuff, just use JX.Workflow so we get exception
dialogs. We have plans to enhance these anyway (see T302).
Test Plan: Changed the changeset view controller to throw exceptions. Verified I
got un-mysterious exception dialogs when a changeset failed because of an
exception in either initial rendering or after hitting "see more".
Reviewed By: tomo
Reviewers: jungejason, tuomaspelkonen, aran, tomo
CC: aran, epriestley, tomo
Differential Revision: 679