Check _abortException before checking _shutdown flag (#56552)

Check _abortException before checking _shutdown flag (#56552)

In case of an error aborting the connection, there is a race between a thread creating new `Http2Stream` to send a request and the thread looping in `ProcessIncomingFrames` that sets _shutdown flag and `_abortException`. If the request thread first sees `_shutdown == true`, then it won't see the `_abortException` even if it's set, so the request will be retried when it shouldn't.

This PR adds `_abortException` check just before the `_shutdown == true` check to make sure an abort exception is observed.

Fixes #1581
Fixes #56138
Fixes #56026