Add coverage to UpdateMailTest for NOT_FETCHED and other error conditions

Problem/Motivation

#3285176: Warning message links to "available updates" even if user does not have permission for that page had some broken logic around links in emails. The test bot didn't complain, since at #3538637: Fix problems with UpdateMailTest: only test valid cases, use correct paths, remove dead code we never added test cases for situations like REVOKED, NOT_FETCHED, UNKNOWN, etc.

Steps to reproduce

Proposed resolution

Add more cases to \Drupal\Tests\update\Unit\UpdateMailTest::providerTestUpdateEmail()

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet