Capturing Return_Value From SP And Displaying Message On The Page Based On The Value

Hello

I'm using FromView to Insert record into a DB. My SP checks for the existence and returns -1 if record already exists. I know that my SP returns -1 but how do I capture that into FormView and display error message to the user? All I get is the "Agreement was created" regardless of what SP returns.
Your help is appreciated.

Mark
Here is the code I'm using:
Protected Sub OnInserted(sender As Object, e As System.Web.UI.WebControls.SqlDataSourceStatusEventArgs)
        Dim retValParam As New SqlParameter("@RETURN_VALUE", SqlDbType.Int)
        retValParam.Direction = ParameterDirection.ReturnValue
        
 
        Dim valParam As Integer = Convert.ToInt32(retValParam.Value)
        If valParam = -1 Then
          lblMessage.Text = "The Account ID is not valid"
        Else
        if valParam = -2 then
          lblMessage.Text = "Agreement ID already exists"
          else
          lblMessage.Text = "Agreement was created"
        End If
      end if
  End Sub

and parts of ASPX page with FromView:

 InsertCommand="spNew_Agreement" OnInserted="OnInserted">
 
  <FilterParameters>
  <asp:ControlParameter Name="Acct_Name" ControlID="txtAcct_Name" />
  </FilterParameters>
  <InsertParameters>
  <asp:Parameter Name="acct_id" Type="string" />
  <asp:Parameter Name="agreement_id" Type="string" />
  <asp:Parameter Name="return_value" Type="int32" Direction="ReturnValue" />
  </InsertParameters>