There's a version of this that keeps the PSBT in memory and does some
fancy addition/subtraction of unuseable parts for the v2's, however
it's much easier and simpler to simply error on the peer and re-start
from the very beginning.
This only works if we haven't gotten commitments from the peer yet (in
fact either method would only work if we haven't got commitments from
the peer yet), so if we've got commitments from them we simply mark them
as failed an go again.
In a perfect world, we'd remember what inputs we used last time, and
reuse those again on the re-attempt, which would pefectly guarantee both
that the failed opens (ones w/ commitments exchanged) would be canceled
after this completes (and we could re-try the failed again).
As it is, this is not perfect. It is, however, servicable.
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