rescue Addressable::URI::InvalidURIError with proper error message

I noticed the default input for pages#index, 'http://', throws a 500 error (see:
http://discovery.my.usa.gov/pages?url=http%3A%2F%2F). I added a rescue clause to
render a more informative error message using json.

I also added a spec to test for malformed url.  I'd like to test against
additional error prone URLs but this should suffice for now.