Detect Add-in

I need to send a file to hundreds of users so that I can tell who has a particular add-in installed. This file will then be saved to a particular directory. The problem I'm having is controlling the error that occurs when the add-in is NOT installed. The below approach (using On Error Goto) clearly isn't the way to go. I need something like
If(Iserror(AddIns("AmartaSpecs") Then Cells(2,3)="No"

The code I have so far is:


Sub Amarta_Specs()
Dim a

'ActiveSheet.Shapes("Button 1").Delete
Cells(1, 1) = "Username"
Cells(1, 2) = "Computer Name"
Cells(1, 3) = "Add-in installed?"

Cells(2, 1) = Environ("Username")
Cells(2, 2) = Environ("Computername")

On Error GoTo Error
Set a = AddIns("AmartaSpecs")
Cells(2, 3) = "Yes"

Error:
Cells(2, 3) = "No"

ChDir "E:FinanceSQL Add-in test"
ActiveWorkbook.SaveAs Filename:="E:FinanceSQL Add-in test" & Environ("Username") & ".xls"

End Sub