r.geomorphon: suggest most likely option in the error message

Not using std G_option_required() function because all options are in a long list here.


git-svn-id: https://svn.osgeo.org/grass/grass/trunk@71792 15284696-431f-4ddb-bdfa-cd5b030d7da7

raster/r.geomorphon/main.c

@@ -198,7 +198,8 @@ int main(int argc, char **argv)
 		num_outputs++;
 	    }
 	if (!num_outputs && !multires)
-	    G_fatal_error(_("At least one output is required"));
+	    G_fatal_error(_("At least one output is required, e.g. %s"),
+		      opt_output[o_forms]->key);
 
 	meters = (flag_units->answer != 0);
 	extended = (flag_extended->answer != 0);