We do this (send warnings) in almost all cases anyway, so mainly this
is a textual update, but there are some changes:
1. Send ERROR not WARNING if they send a malformed commitment secret.
2. Send WARNING not ERROR if they get the shutdown_scriptpubkey wrong (vs upfront)
3. Send WARNING not ERROR if they send a bad shutdown_scriptpubkey (e.g. p2pkh in future)
4. Rename some vars 'err' to 'warn' to make it clear we send a warning.
This means test_option_upfront_shutdown_script can be made reliable, too,
and it now warns and doesn't automatically close channel.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
2526e804f7