Return a HTTP 500 instead of a HTTP 400 if an internal error occurs in the Aphlict server

Summary: Ref T5651. Only throw a HTTP 400 if the data is invalid (i.e. the request is bad). If something bad happens when trying to transmit the notification, throw a HTTP 500 instead.

Test Plan: Eye-ball it.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T5651

Differential Revision: https://secure.phabricator.com/D9968