Better error messages on satisfiers

I've been learning rust-miniscript, and when attempting to try out timelocks, I kept hitting a CouldNotSatisfy when finalizing my psbt. The reason turned out to be totally of my own doing - I had inadvertantly used a Sequence::MAX instead of a Sequence::ZERO when constructing my spend transaction. I had effectively disabled locktime usage, because I had done this:

 let txin = TxIn {
        previous_output,
        ..Default::default()
    }; 

instead of this:

 let txin = TxIn {
        previous_output,
        sequence: Sequence::ZERO,
        ..Default::default()
    }; 

I was so preoccupied trying to figure out how signing worked, how to use the plan module etc that I didn't notice this.

In order to figure out the problem, I ran my own local forks of bdk_wallet, rust-miniscript, and bitcoin crates, instrumenting them with printlns until I figure out what the (dumb, self-inflicted) problem was.

It would be supersonic if satisfier errors gave a more specific reason for failure - I wonder if it would be possible for instance to turn CouldNotSatisfy into something like CouldNotSatisfy("here is the reason")? Or perhaps something more typed like (in the case I just dealt with) CouldNotSatisfy(LockTimeDisabled)?

Start by tracing the satisfier errors around CouldNotSatisfy, especially when finalizing a PSBT and using the plan module. Reproduce the timelock case with Sequence::MAX versus Sequence::ZERO, then determine how failure reasons should be represented. Done means callers receive a more specific, actionable explanation for satisfier failures.