Here's the general syntax:

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As _
System.EventArgs) Handles Button1.Click
   Try
      Process.Start("http://www.microsoft.com")
   Catch ex As Exception
      MsgBox("Can't load Web page" & vbCrLf & ex.Message)
   End Try
End Sub

If you are making a web app, instead of MsgBox just drop a Label on your page, and set it's Text property to a string that summarizes what the error is about in user-friendly terms. So, you'd have something like this:

Catch ex As Exception
      Label1.Text = "You can't vote twice"
End Try