Add custom inspect to Google::Apis:Error (#729)

Add the status_code, header, and body values to inspect when present.
These values can help users understand why they are getting errors,
but many users don't know to look for them.
Adding the values to inspect should help discoverability.

lib/google/apis/errors.rb

@@ -41,6 +41,15 @@ module Google
           super
         end
       end
+
+      def inspect
+        extra = ""
+        extra << " status_code: #{status_code.inspect}" unless status_code.nil?
+        extra << " header: #{header.inspect}"           unless header.nil?
+        extra << " body: #{body.inspect}"               unless body.nil?
+
+        "#<#{self.class.name}: #{message}#{extra}>"
+      end
     end
 
     # An error which is raised when there is an unexpected response or other