Basic error handling for API responses added

I saw that error-handling for the project is still TBD. 

This pull request does not handle all and any kind of errors, but at least it throws some additional info if the API responds with HTTP 400 or 404 or 500. 

Basically, I had a malformed list of init parameters and was getting "calling .map() on undefined" instead of a helpful message, so I think it would help in similar cases :)

If you could merge the pull and re-publish npm, it would be super!

Thanks for the project!