The bug was in ein:query-ajax-callback. When url-retrieve called in
its callback (which is the case of ein:notebooklist-delete-notebook),
same process buffer is reused. Therefore, old code killed another
buffer (namely notebook list buffer) by accident and the variable
url-http-response-status was void because it is outside of the process
buffer (I am not sure about this part). Current workaround is to
check if the buffer is live before executing buffer, using new macro
ein:with-live-buffer.
There was two bugs:
* Process buffer was killed in ein:notebooklist-url-retrieve-callback
where should be handled by ein:query-ajax-callback. There was two
calls of kill-buffer. This is why the list buffer was killed.
* At the time ein:query-ajax-cancel-timer is called in
ein:query-ajax-callback the current buffer was changed because
pop-to-buffer is called in the callback. Wrapping callbacks
with save-current-buffer fixes the problem.
