Improve visibility of repository credential errors

Summary:
Fixes T7310. We have a whole mechanism for surfacing update errors, but only surface actual update errors, not pull errors.

Instead, surface pull errors too.

Then format them a little more nicely.

Test Plan:

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7310

Differential Revision: https://secure.phabricator.com/D11821