Add meaningful error messages

[atimmer]

I was bitten by [sprintf] huh?, this should make debugging for other people much easier.

[atimmer]

Another possibility would be depending on https://github.com/alexei/sprintf.js which looks at lot like the inline sprintf-implementation.

[SlexAxton]

It is the alexei implementation, just inlined. Feel free to just re-inline the latest.

[atimmer]

Wouldn't it be better to depend on it in the package.json? This means updates in the future will be easier to incorporate. I am willing to do this work.

[SlexAxton]

Happy to take a PR. It works like it currently does because I released Jed in 2012 and sprintf wasn't on Github until 2013. It was just a file on his website.