R1:9e6507d37aef

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…