Don't disrupt repository import chain when publishing is disabled

Summary:
Fixes T4736. Currently, we incorrectly skip the writeImportStatusFlag() call if publishing is disabled (the herald-disabled) check. This means we don't flag the commit as imported, and don't move the pipeline forward correctly.

Instead, we only want to skip the owners stuff, not the pipeline stuff. Move that to a method.

(Also fix a nearby TODO now that we have a permanent failure exception.)

Test Plan:

• Used scripts/repository/reparse.php --owners ... to execute this code, fiddled with things to hit both the disabled and enabled branches and verified the flag stuff is still reached.
• Faked the exceptions and made sure they raise correctly.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4736

Differential Revision: https://secure.phabricator.com/D8715