[native] Show "unknown error" alert if keyserver auth fails during registration

Summary:
We display this "unknown error" alert when identity auth fails, but for keyserver auth we're just failing silently. I think it's best to be consistent and show some alert indicating the error.

Depends on D11846

Test Plan: Confirm the Alert displays

Reviewers: varun

Reviewed By: varun

Subscribers: inka, tomek

Differential Revision: https://phab.comm.dev/D11847