Use this to test the pubkey paths in checksig which return btcec errors
which we don't define. all of the other active tests know the return
code we need.
Use it to reduce code dpulication in the bip16 case.
In addition we export it so that that a user could run:
for !done && err == nil {
done, err = s.Step()
}
err = s.CheckErrorCondition()
manually instead of having to run Execute().
