'Class1.cls
Public Sub Hoge()
Err.Raise vbObjectError + 1001
End Sub

'Form1.frm
Private Sub Command1_Click()
On Error GoTo myerror
Dim c As New Class1
Call c.Hoge
Exit Sub
myerror:
Dim e
e = Err.Number - vbObjectError
MsgBox "Errorだよ" & e
End Sub