Don't fatal on duplicate field in commit message

Summary:
If the actual commit message has a duplicate field and we shouldAutoclose it then the commit message parser fails.
Put the error in $errors instead.

Test Plan:
Reparsed commit with duplicate field in message.
Tried to arc diff message with duplicate field.

Reviewers: epriestley, nh

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3470