Make "Land Revision" show merge conflicts more clearly
Summary:
Ref T182. We just show "an error happened" right now. Improve this behavior.
This error handling chain is a bit ad-hoc for now but we can formalize it as we hit other cases.
Test Plan:
{F910247}
{F910248}
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T182
Differential Revision: https://secure.phabricator.com/D14343