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