noreturn for liboctave error handler function pointers

[John W. Eaton]

Rik:

I agree that tagging the liboctave error handlers with "noreturn" is the 
right thing to do, but in the past they did return (or at least the ones 
we defined in Octave did return).

I suppose it's possible that someone provided their own versions of 
these functions if they were using liboctave in their own program that 
also returned after printing a message.

I guess as things are, liboctave functions can still work properly if 
the function called through the liboctave_error_handler pointer returns 
because that's the way things have always worked up until now.  But 
things will break if we start modifying liboctave in the same way as we 
have modified the libinterp sources.

Should we care?  Maybe just document the change in NEWS?

jwe