You'll want to wrap your code in "On Error Resume Next", then handle the error.
On Error Resume Next
Set field = custFields.Field("TEST", "TS_USER_88")
If Err.Number <> 0 Then
'Do Something to handle your error
'stuff
'Clear the error
Err.Clear
End If
On Error Goto 0
'more stuff down here
Here's some more info on the Err object and some of it's properties:
http://msdn.microsoft.com/en-us/library/sbf5ze0e(v=vs.84).aspx