How Do I Identify This Error?

I have a control (command button) on a form that opens up a report which filters out a single record. The filter is based upon a value in a text box located on the form. Within the VB code, I put some code to prevent the form from opening if the text box had no data in it.

When i click on the button (assuming the text box has a value in it), I get the error message I defined, and the report opens properly. This tells me there is an error being generated.

My question is, how I determine what error has been generated? And how do I fix it?

Here is the code I used:

//This is the code for the form.
Private Sub cmdButtonPreviewLabel_Click()

On Error GoTo Form_Closure_Error2
DoCmd.OpenReport "reportLabels", acViewPreview

Form_Closure_Error2:
MsgBox "Sorry, but I cannot print a label without a Job Number.", vbOKOnly, "Notice"
Exit Sub


//This is the code for the report
Private Sub Report_Open(Cancel As Integer)

On Error GoTo Form_Closure_Error1

tempvalue1 = Forms![frmjoblog].[JobNumber]
Me.Filter = "[tableJobLog].[JobNumber]= " & tempvalue1
Me.FilterOn = True

Form_Closure_Error1:
Exit Sub

End Sub

End Sub