Include Error Handling section in README

[yjukaku]

As of now, I have no idea if/how this excellent gem handles errors. Documentation about exceptions, return values, etc. would really be helpful.

[stevenharman]

@yjukaku You are quite correct. We have added a wrapper for feedback errors (see #51), but it's not been documented. I'd be happy to accept such documentation and enhancements to make dealing with errors easier for users.

Thank you.