Jetty 12 should throw ISE on sendError/reset/resetBuffer after flushBuffer being called

unit test added in branch `jetty-12.0.x-should-throw-ise-after-flush` but unit `ErrorPageTest` get stucked after the first attempt of fix

Start by reviewing the ErrorPageTest unit and the `jetty-12.0.x-should-throw-ise-after-flush` branch, then reproduce where the test becomes stuck after the first attempted fix. Done means sendError, reset, and resetBuffer throw ISE after flushBuffer has been called and ErrorPageTest completes.

Written by the indexing model from the issue text.