>You can trap any error
>ON ERROR DO processerror()
>and replace it with custom on, but in most cases you can just remove the source of error, e.g.
>IF USED("mytable")
>Select mytable
>Endif

An error handler doesn't stop the rather annoying behavior of presenting the user with an open table dialog when a line or SQL statement references an alias that is not open. The only way I know of to do this is just make darn sure your tables are opened before running the line.