Improve Drydock errors for empty commits and missing changes

Summary: Ref T10093. Show better errors when a commit fails because it has already been merged and when a fetch fails because the ref isn't present in the remote.

Test Plan:

Reviewers: chad

Reviewed By: chad

Subscribers: michaeljs1990, yelirekim

Maniphest Tasks: T10093

Differential Revision: https://secure.phabricator.com/D15420