Use the new Debug.log_backtrace function

The rules are
- if you are the last person who will see the exception (e.g.
  you are handling it) then you may log the backtrace if that will
  be interesting to anyone
- if you are not the last person who will see the exception,
  for heaven's sake don't spam the log with it

Signed-off-by: David Scott <dave.scott@eu.citrix.com>