Return partial differential fields when there's an error parsing

Summary:
If someone typos one name in a cc or reviewer list, it would be nice if we
display all of the valid names in the field when running arc diff (in addition
to the error message).

Test Plan:
used the conduit console to check that calling differential.parsecommitmessage
with a list of some valid and some invalid ccs returns a result with both an
error and a list of some ccs. Also ran arc diff with that list of ccs to check
for the correct user experience.

Reviewers: epriestley, jungejason, vrana

Reviewed By: epriestley

CC: aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2542