You've already got an If clause that checks if it worked (i.e. an appointment was created) so you just need, as you said, to add a message to the Else part to indicate it didn't work, and the only real info about that you will have is from the error object, so you could do something like:

If objAppt Is Nothing Then
   Msgbox "Failed to create appointment!" & vbcrlf & "Error: " & err.description
else
' run your code
End If