Fix exception handling in ConpherenceViewController::handleRequest()

Summary:
It is only possible throw an Exception class. Aphront400Response is not an Exception class but a Phobject.
Thus return it, like the Aphront404Responses within the same method.

Test Plan:
Check parent classes; run static code analysis. Hack the code to force that return,
and successfully obtain an HTTP 400 status code page.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25780