Raise error on bad response data

When trying to sign response make sue the data is correct and raise a known error rather than a method missing error on bad data.

lib/u2f/sign_response.rb

@@ -4,6 +4,10 @@ class SignResponse
 
     def self.load_from_json(json)
       data = ::JSON.parse(json)
+      if data['clientData'].blank? || data['keyHandle'].blank? || data['signatureData'].blank?
+        raise Error, 'Missing required data'
+      end
+
       instance = new
       instance.client_data_json =
         ::U2F.urlsafe_decode64(data['clientData'])