Clean up handling of DAP not-stopped response

This patch introduces a new NotStoppedException type and changes the
DAP implementation of "not stopped" to use it.  I was already touching
some code in this area and I thought this looked a little cleaner.
This also has the advantage that we can now choose not to log the
exception -- previously I was sometimes a bit alarmed when seeing this
in the logs, even though it is harmless.

Reviewed-By: Kévin Le Gouguec <legouguec@adacore.com>