bug#38384: (next|previous)-buffer silent about not switching

[martin rudalics]

> After a cursory glance, I find only one instance of (next-buffer) called
> from elisp in our sources, and it's in mode-line-next-buffer.  And in that
> case, clicking on the buffer name in the modeline and getting a message "no
> next buffer" seems like an improvement to me over the silent treatment ;-)

Right again.  The same holds for 'mode-line-previous-buffer'.

martin