VBA error handling query

I am not quite sure I understand your objective. If I did, I would probably disagree.

Is this code you are developing? I almost never use error handling in code I am developing. I want the interpreter to stop on the statement that gives the error. I want to understand why that error has occurred. What could I have done to avoid the error? Did I fail to check the file exists? Did I fail to check the path is accessable? I will add the missing code before I do anything else.

By the time you have finished development, you should plan for there to be no error condition for which you have not included proper code. Of course, this is impossible; you cannot make your code foolproof because fools are so ingenious. The version you release to users must contains error handling.

But you could not release this code to users since it would stop without warning. Would the user guess something had gone wrong with the macro or would they assume this was what was supposed to happen? If they decide the macro has failed what are they going to say to you? "It did not do what I was expecting and I do not know why." What are you going to say back? "What were you doing?" I do not think I have ever had a user give a believable description of what they were doing at the time of a failure. At the very least you want:

Call MsgBox("Sorry I have had an unrecoverable error within QOScode()." & _
            " Please record: " & Err.Number & " " & Err.Description & _
            " and report to extension 1234")

With this, the user does not wonder if something has gone wrong and you know where it went wrong and, with luck, why.

To handle the foreseen issue neatly you can use short error handling to test that the Workbook actually exists (ie If Not Wb Is Nothing Then, if so work on it, with a common ending (ie destroying the object)

The second sample shows how to add additional handling for unforeseen errors once the workbook has been opened.I have used Err.Raise to create deliberate error to give the user a choice as to how to proceed (close the workbook immediately post error or make the workbook visible)

As an aside, don't use Dim and New together. I have re-written
Dim app As New Excel.Application
into
Dim xlApp As Excel.Application
Set xlApp = New Excel.Application

1. Handling the no workbook issue

Sub QOScode()
    Dim xlApp As Excel.Application
    Set xlApp = New Excel.Application
    Dim Wb As Excel.Workbook
    On Error Resume Next
    Set Wb = xlApp.Workbooks.Add(ActiveWorkbook.Path & "\QOaS DGL stuff.xlsx")    'this sheet does not exist
    On Error GoTo 0    '

    If Not Wb Is Nothing Then
        MsgBox Wb.Sheets("ACLS").Cells(3, 3)
        'Do what you have to do
        Wb.Close False
    End If

    xlApp.Quit
    Set xlApp = Nothing
End Sub

2. Handling no workbook and other unforseen error which may otherwise leave the worbook open

Sub QOScode2()
    Dim xlApp As Excel.Application
    Set xlApp = New Excel.Application
    Dim Wb As Excel.Workbook
    Dim lngChk As Long
    On Error Resume Next
    Set Wb = xlApp.Workbooks.Add(ActiveWorkbook.Path & "\QOaS DGL stuff.xlsx")    'this sheet does not exist
    On Error GoTo 0    '

    If Not Wb Is Nothing Then
        On Error GoTo ProblemHandler
        MsgBox Wb.Sheets("ACLS").Cells(3, 3)
        'Do what you have to do

        'Deliberate error
        Err.Raise 2000, "test code", "Sample Error"
        Wb.Close False
    Else
        MsgBox "Workbook not found,code will now exit"
    End If

    xlApp.Quit
    Set xlApp = Nothing
    Exit Sub

ProblemHandler:
    'Test to see if workbook was still open when error happened
    If Not Wb Is Nothing Then
        lngChk = MsgBox("The code encountered an error" & vbNewLine & Err.Number & vbNewLine & "Do you want to close the file?", vbYesNo, Err.Description)
        If lngChk = vbYes Then
            'close book. Code will proceed to destroy app
            Wb.Close False
        Else
            'make workbook visible and leave code
            Wb.Visible = True
            Exit Sub
        End If
    Else
        MsgBox "The code encountered an error - the file was already closed at this point" & vbNewLine & "Error number " & Err.Number & vbNewLine, Err.Description
    End If
    'destroy app (either if the workbook was closed, or user chose to close it)
    xlApp.Quit
    Set xlApp = Nothing

End Sub