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.
Reviewed By: chad
Subscribers: michaeljs1990, yelirekim
Maniphest Tasks: T10093
Differential Revision: https://secure.phabricator.com/D15420