Catching Validation Error in ASP.Net

Trying to figure out where to catch validation exception.

I have a asp.net web form that has a formView control that is connected to a LinqDataSource control.  Created a partial class to do the validation (see below).  When the exception is thrown I cannot figure out where to catch it.

I expected to find and event on the LingDataSource control.   I am new to using databound controls/data source controls so I am hoping this is a very easy.

Partial Public Class User

    Private Sub OnUser_NameChanging(value As String)
        If value.Contains("jon") Then
            Throw New Exception("jon is not valid")
        End If
    End Sub

End Class

Select allOpen in new window