fix(ci): fail check-plugin-provides --write on an unreadable plugin

[datlechin]

Why

scripts/ci/check-plugin-provides.py --write printed stamped OddDriverPlugin: declares neither databaseTypeId nor formatId as a string literal and exited 0 for a plugin it could not read, leaving its Info.plist unchanged. The unreadable case went into the same list as the key mismatches, and --write printed that whole list as stamped. Follows #3502.

Change

• problems_in raises Unreadable instead of returning it as a mismatch.
• --write prints stamped <Plugin>/Info.plist only for plists it rewrote, prints each unreadable plugin as an ::error:: and exits 1.
• The check run offers --write only when a mismatch is left for it to fix.

Verified

• test_write_fails_on_an_unreadable_plugin_and_leaves_its_plist fails on main and passes here; the full scripts/ci suite passes on Python 3.9.
• The three unreadable cases in test_check_plugin_provides.py now expect Unreadable to be raised.