Terminate keep-alive in kill-buffer-hook.

Since lyskom-keep-alive-timers is a buffer-local variable,
lyskom-keep-alive-callback can't stop the keep-alive timer
if the buffer has been killed.  As a result, it keeps saying ERROR! in
the mini buffer.

On an idea from Christer Ekholm, terminate the timer explicitly with a
kill-buffer-hook instead.  The error logging in
lyskom-keep-alive-callback has been improved too, to provide more
information on what went wrong if there are other error situations in
the future.