fix issue with --satpoint when offset > 0. (Fix #2465)

[felipelincoln]

the prevouts being used to construct the reveal signature is:

let mut prevouts = vec![unsigned_commit_tx.output[0].clone()];

if let Some(parent_info) = parent_info {
  prevouts.insert(0, parent_info.tx_out);
}

but using unsigned_commit_tx.output[0] as one of its elements is a wrong assumption since a inscribe run can be called using --satpoint with offset > 0, which would make the first output of the commit transaction to be a change (part not used of the satpoint), and not the output encoding the taproot spending condition.

Example of a inscribe using --satpoint=cad80aa153050fa300fc539e35fbb2046f4c99d3e09b27437db2232bfec43f71:2:1000

(example produced building ord with the code from this pr. https://mempool.space/pt/testnet/tx/aab6b689ec23b60e0943adf6357eb3a2cdb8c50ebfaf10cb972f96155a239f4b#flow=&vout=1)
In this case, unsigned_commit_tx.output[0] is the first output with 1000sat value

---

this pr fix #2465

[raphjaph]

Nice, good catch! I added an error message that is more helpful in case someone runs into issues in the future.

Pro tip: Create a feature branch on your repo with the changes and open a PR with that. Makes development for you easier