Jetty 12: Review asyncError handling

[sbordet]

Jetty version(s)
12+

Jetty Environment
ee10

Description
After #10949, ServletChannelState.asyncError() is now invoked as part of calls to onFailure() since it is invoked through a failure listener.

However, I think this is a bit too much, as asyncError() was initially designed for client-side errors delivered via HTTP/2 RST_STREAM.

Now, since it's invoked via the onFailure() mechanism, it is invoked also in many other cases, often producing IllegalStateException -- a signal that it may not be right to invoke it.

We should review whether restore the original semantic for asyncError() (only invoked for RST_STREAM), or modify it for the new one (always invoked for onFailure()).

[sbordet]

This has been addressed.

It was deemed ok to always invoked asyncError() in case of asynchronous failures, of which HTTP/2 RST_STREAM is only one case.