Don't show error operations after a successful land operation

Summary:
Ref T182. When viewing a revision, if there are several error operations and then a success operation, we currently show the last error. This is misleading.

Instead, don't show anything if there's a success (this may require tuning eventually if you can land multiple times onto different branches or whatever, but should be reasonable for now).

Also make the table a little nicer, particularly for merge failure output.

Test Plan:

Reviewers: chad, Mnkras

Reviewed By: Mnkras

Maniphest Tasks: T182

Differential Revision: https://secure.phabricator.com/D14348