Refactor the throws in aria-utils

[cookiecrook]

[the throw messages in aria-utils] only a help for authors while writing the test, and a classname typo would still result in a silent failure unless all of them had the same typo, so @annevk pointed out they have limited utility.

Originally posted by @cookiecrook in web-platform-tests/wpt#43740 (comment)

Perhaps there is a better way to approach this.

[cookiecrook]

Actually in discussing with @rahimabdi today, any necessary changes will probably just be worked into @jugglinmike's PR for

• [accname] Require specification of subtest count wpt#42769