Add a proper error to Bitcoin's SignableTransaction::new

Also adds documentation to various parts of bitcoin.

crypto/frost/src/tests/mod.rs

@@ -58,7 +58,7 @@ pub fn algorithm_machines<R: RngCore, C: Curve, A: Algorithm<C>>(
     .iter()
     .filter_map(|(i, keys)| {
       if included.contains(i) {
-        Some((*i, AlgorithmMachine::new(algorithm.clone(), keys.clone()).unwrap()))
+        Some((*i, AlgorithmMachine::new(algorithm.clone(), keys.clone())))
       } else {
         None
       }