Throw real errors instead of strings

[rrrnld]

I would suggest throwing real errors instead of strings because they provide additional information helpful for debugging. This appears to be a rather minor change with a decent benefit and if there's. interest in this I can write the code and open the pull request. Any thoughts?

[fpirsch]

This project is long dead. I suggest using live forks, like https://github.com/fpirsch/twin-bcrypt or https://github.com/dcodeIO/bcrypt.js