scripts/qapi/backend: Clean up create_backend()'s failure mode

scripts/qapi/backend: Clean up create_backend()'s failure mode

create_backend()'s caller catches QAPIError, and returns non-zero exit
code on catch.  The caller's caller passes the exit code to
sys.exit().

create_backend() doesn't care: it reports errors to stderr and
sys.exit()s.

Change it to raise QAPIError instead.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20250311065352.992307-1-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>