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:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T182

Differential Revision: https://secure.phabricator.com/D14343