add backtrace printing to elnode-errors

[nicferrier]

Here's is how to do backtrace collection:

(defun nic-test-2 ()
  (let ((standard-output (get-buffer-create "*nic-backtrace*")))
    (backtrace)))

An initial thought is that any condition-case capture could capture the backtrace in a unique buffer and add a link to the produced error message to elnode-log that will let you follow it. But that might be expensive on buffers.

Another alternative would be to add it as a property to the log-line so that a buffer viewer mode could handle it's display