improve nargin error message (bug #47021) * ov-usr-fcn.cc (Fnargin): Pass true to octave_value::function_value. Display type name in error if object is not a user-defined function.

%!function r = f (x)
%!  if (ischar (x))
%!    r = x;
%!  else
%!    error ("expecting character string");
%!  endif
%!endfunction

%!assert (eval ("f 10;"), "10");