Fixing a test for None on both variables in body and html

If body variable didn't exist and html did, pyramid_mailer was assuming both were empty, note that I only discovered this when moving from message.send() to message.send_to_queue(), but nevertheless this fix does the same thing as before, but this actually is a true test on both variables being empty.
My fix tests to see if 'body' and 'html' are both empty and then returns an error, the previous code was throwing an error message when one of these was NOT None/empty. The new fix I added tests both variables to see if they are empty, and if so, returns an error.