Raise a more tailored error message when a third-party test engine returns bad results

Summary: Fixes T8714. When a test engine isn't returning the correct result type, shift suspicion onto it.

Test Plan: Faked error, got exception blaming test engine.

Reviewers: joshuaspence, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8714

Differential Revision: https://secure.phabricator.com/D13486