Improve UX and messaging for certain errors when landing revisions

Summary:
Ref T9994.

• Allow errors to be dismissed.
• Tailor messaging for closed/abandoned revisions.
• Reduce scare messaging on land dialog, since it's not really that scary anymore.

Test Plan:

• Dismissed errors.
• Hit new warnings.
• Wasn't as scared when landing.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9994

Differential Revision: https://secure.phabricator.com/D14886