Don't panic trying to spend outputs with the wrong scripts

[TheBlueMatt]

We currently panic in spend_spendable_outputs if the script doesn't match, which is a bit strange given we tell users explicitly they should override the coop close scripts. Instead we should have an error enum.

We should also much more clearly document how to handle spendable outputs events with overridden shutdown scripts. It's mentioned in the spendable enum variant itself but this isn't at all discoverable looking at the event.

[TheBlueMatt]

Turned out to be complicated and 116 has too much else going on.

[TheBlueMatt]

Oops left this comment on the wrong issue. Anyway, we should do this, and while its not complicated, it doesn't have to happen in 116.

[TheBlueMatt]

Also when we do this we need to de-dup the descriptors rather than erroring on dups.