txscript: rename scriptError to makeError.

[dnldd]

This rename keeps error construction consistent across modules while confirming to the error handling infrastructure guidelines outlined in #2181.

[davecgh]

I'm not really a fan of this change to be honest. There are different types of errors that can come out of the txscript package. I like that it is explicit.

[dnldd]

The ErrorKind has the details of the type of error being created and that is present with makeError as well, I think the consistency across modules with error creation is more beneficial here. Do reconsider. Also different types of errors can be generated from txscript would it not be better to let the ErrorKind show that via makeError Instead of adding new constructors for the different error types added?

[dnldd]

After further discussion, my change does not explicitly show the class of error being created, only the error kind. I'll be closing this PR and reverting the changes made to the error helpers in other PRs as a result.