Making tests run via rake and new GoogleSiteSearch::Error

First of all thanks for this great gem. Saved me a ton of headache dealing with google's API. Super simple to set up and seems to work well.

Here's a small patch to make `rake test` run. Also I added `GoogleSiteSearch::Error` so it can be thrown instead of `StandardError`. Didn't want to do pokemon rescuing in my code.

Thanks again!