In "bin/repository reparse", continue on "PhabricatorWorkerPermanentFailureException"

Summary:
Fixes T13315. See that task for discussion.

Without --background, we currently treat this as a catastrophic failure, but it's relatively routine for some repository states. We can safely continue reparsing other steps.

Test Plan: Ran bin/repository reparse --all X --message with commits faked to all be unreachable. Got warnings instead of a hard failure on first problem.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13315

Differential Revision: https://secure.phabricator.com/D20588