eliminate more simple uses of error_state * dasrt.cc, debug.cc, find.cc, gammainc.cc, matrix_type.cc, ov-usr-fcn.cc, pt-assign.cc, pt-binop.cc: Eliminate simple uses of error_state.

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

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