Don't send error message to stdout, but to stderr.

src/frontend/inp.c

@@ -1305,10 +1305,10 @@ inp_dodeck(
 
                 if (p == dd->error) {
                     if (strstr(dd->line, ".model"))
-                        out_printf("Warning: Model issue on line %d :\n  %.*s ...\n%s\n",
+                        fprintf(stderr, "Warning: Model issue on line %d :\n  %.*s ...\n%s\n",
                                    dd->linenum_orig, 72, dd->line, dd->error);
                     else {
-                        out_printf("Error on line %d or its substitute:\n  %s\n%s\n",
+                        fprintf(stderr, "Error on line %d or its substitute:\n  %s\n%s\n",
                                    dd->linenum_orig, dd->line, dd->error);
                         have_err = TRUE;
                         return 1;