Use verbatim commit message if it couldn't be parsed

Summary:
We currently remove an invalid user from Reviewers and similar fields and print a note in the template. There are several problems:

1. If I only made a typo in the name then it takes some effort to fix this typo and put the name on the original place (this information is lost).
2. The notice is almost invisible and most users will just confirm the message without noticing it.
3. If I fill the data in the commit message (and not in the template) then I probably want to treat that as authoritative so I want it to be correct.

Test Plan:
arc diff with invalid reviewer in commit message.
arc diff on empty commit message.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin, nh

Differential Revision: https://secure.phabricator.com/D2730