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Previously, the auth error handling logic existed twice, depending on
whether the initial setup was finished or not. This CL deduplicates the
code.
This causes two small behavior differences:
1) Before, a TWO_FACTOR auth error was treated as "okay" if first-time
   setup was incomplete, but treated as an error otherwise. That just
   seems like an oversight.
2) Before, if there was an auth error while we were also missing the
   Sync confirmation, we'd show a "missing confirmation" message. Now
   we'll show "auth error" instead.

This CL also relaxes the nullable-restrictions on output params:
|status_label|, |link_label| and |action_type| can now all independently
be null or not.

Bug: 911153
Change-Id: Ic2e5174c2596114725f6816d4f385ebf5c50ece6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1507872
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Mohamed Amir Yosef <mamir@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638582}