I've just tried out agatha and it does what I need for now and is dirt symple to set up - recommended. However, out of the box it breaks with PHP5 with the error message "Fatal error cannot redeclare error()..." (I don't have the exact error, since I've got a workaround).
The workaround for this is to wrap the error() declarations in index.php and agatha.php with the PHP snippet:
# existing error() function goes here...
I can upload a patch if you like. Let me know.