Tweak logic for when extension txt is latest

Always use the latest extension as the latest API, regardless of
the current base extension version. This makes it so that we can
bump the base in master without changing which txt is considered
latest.

Also extend the error check to apply more widely (which caught an
error in the extension 3 finalization).

Bug: 228017107
Test: m checkapi (with api break & bumped base)
Test: prebuilt_apis_test.go
Change-Id: Ia10eb41ff500c566736f0d29a29984a99386bb6f