scripts: run_common: don't dump stack on unavailable runner

Originally reported in #23539 (though that seems to have been another
problem), west flash and friends are dumping stack when used with an
unconfigured runner.

Let's just promote the warning about this to an error. The idea that
this ever could have worked without explicit support has not worked
out in practice, to my knowledge.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>