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 `Aphront404Response`s 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