v.colors: reading from stdin is not supported, inform user about that

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@65564 15284696-431f-4ddb-bdfa-cd5b030d7da7

vector/v.colors/main.c

@@ -280,6 +280,7 @@ int main(int argc, char *argv[])
 
     Rast_init_colors(&colors);
     if (is_from_stdin) {
+        G_fatal_error(_("Reading color rules from standard input is not currently supported"));
 	/*
         if (!read_color_rules(stdin, &colors, min, max, fp))
             exit(EXIT_FAILURE);