- John Wilkinson filed bug #2155496

  (http://curl.haxx.se/bug/view.cgi?id=2155496) pointing out an error case
  without a proper human-readable error message. When a read callback returns
  a too large value (like when trying to return a negative number) it would
  trigger and the generic error message then makes the proplem slightly
  different to track down. I've added an error message for this now.