R1:61af2e313bfa

Slightly improve friendliness of "no mxmlc" error

Summary: Ref T4324. Add some hints and context.

Test Plan: Read error.

Reviewers: joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Maniphest Tasks: T4324

Differential Revision: https://secure.phabricator.com/D9417