The on ERROR event is for capturing error messages sent by the server, it has nothing to do with errors in mIRC scripts.
There currently isn't any way to generically catch all mIRC scripting errors or otherwise change their appearance. Then again the errors that the OP has posted aren't ones that should ever be seen by the end-user of a script. Scripters should always check if ($dialog(dialogname)) before creating them and any syntax errors like the if one should, of course, be corrected.
For any other kind of errors that the scripter doesn't intend to fix unfortunately the only way to catch them currently is using the :error mechanism (see /help Error handling in mIRC).
