We had special code to fail a forwarded request, but not for an
internally-generated request. Instead, we should pretend the (dead)
plugin responded with a PLUGIN_TERMINATED error, and handle the
request through the normal paths.
This breaks the case where a plugin crashes (or stops itself) in a
hook, so we handle that next.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>