rounding money values, better error Messages

[jacobitosuperstar]

Hello everyone, first of all, thanks for the package, for me has been great inspiration for my work and my learning of Go.

Wanted to ask you guys, if rounding down to 2 decimals, as almost all the monetary values are worked at best at the cents level, is going to be a part of the package eventually or if is something I could do a PR on.

Same with better error messages. As I was learning how to use this stuff and do my own program, I got thrown into the rabbit hole of better error messages which could be something I could also do a PR on.

Just wanted to see if that work would be valuable before doing a PR.

I have this proposals already implemented here, if you may want to take a look.