gh-109276: regrtest: add WORKER_FAILED state (#110148)

Rename WORKER_ERROR to WORKER_BUG. Add WORKER_FAILED state: it does
not stop the manager, whereas WORKER_BUG does.

Change also TestResults.display_result() order: display failed tests
at the end, the important important information.

WorkerThread now tries to get the signal name for negative exit code.