Proper Error Handling Routine

I don't know if this below is "proper", but try this concept:

Option Explicit

Dim MultiPage1 As Object '? Is there a declaration for this variable somewhere in your code ?
Dim Region '? Is there a declaration for this variable somewhere in your code ?
Dim DSSN '? Is there a declaration for this variable somewhere in your code ?
Dim SSMU '? Is there a declaration for this variable somewhere in your code ?

Sub HandleError_1()
    On Error Resume Next
        Dim arr: arr = Evaluate(ActiveWorkbook.Names("L4_Values").RefersTo) ' Is ActiveWorkbook = ThisWorkbook in this case ?
        If IsEmpty(arr) Then MsgBox "Something's wrong here - The End": Exit Sub
    On Error GoTo 0
    
    Dim is_arr As Boolean: is_arr = True
    Dim i&, indx&: If IsArray(arr) Then indx = UBound(arr, 1) Else is_arr = False: indx = 1
    Dim x: x = 0
    Dim y: y = 0
    
    On Error Resume Next
        For i = 1 To indx
            x = MultiPage1.Pages("Page_" & Region).Controls("TextBox_" & Region & "_Disc_" & DSSN & "_" & i).Value
            y = Evaluate("ROUND(SUMIFS(Metric_" & Region & "_SE_MDS,Attribute_PLLevel4,INDEX(L4_Values," & i & "))/" & SSMU & ",1)")
            If Err.Number <> 0 Then x = 0: y = 0 Else GoSub cmp_compare
        Next
    On Error GoTo 0
    
    If is_arr Then Erase arr Else arr = Empty
    
    Exit Sub
    
cmp_compare:
    On Error GoTo 0: On Error GoTo whats_this
        With MultiPage1.Pages("Page_" & Region).Controls("TextBox_" & Region & "_Disc_" & DSSN & "_" & i)
            If x = y Then .BackColor = RGB(223, 243, 249) Else .BackColor = RGB(71, 142, 185)
        End With
    On Error GoTo 0: On Error Resume Next
    Return
    
whats_this:
    Dim info$: info = "Error # " & Str(Err.Number) & " was generated by " & Err.Source & ":" & vbCrLf & vbCrLf & Err.Description
    MsgBox info, vbOKOnly, "Error", Err.HelpFile, Err.HelpContext
    MsgBox "This is THE END", vbOKOnly, " ... :(( ... :(( ..."
    End ' ... :) ... but be careful with "this" line ... :) ...
End Sub

Sub HandleError()
    Dim x As Long
    Dim y As Long
    Dim i As Long
            
    For i = 1 To UBound(Evaluate(ActiveWorkbook.Names("L4_Values").RefersTo))
        On Error GoTo SetValue:
                
        x = MultiPage1.Pages("Page_" & Region).Controls("TextBox_" & Region & "_Disc_" & DSSN & "_" & i).Value
        y = Evaluate("ROUND(SUMIFS(Metric_" & Region & "_SE_MDS,Attribute_PLLevel4,INDEX(L4_Values," & i & "))/" & SSMU & ",1)")
                
        With MultiPage1.Pages("Page_" & Region).Controls("TextBox_" & Region & "_Disc_" & DSSN & "_" & i)
            If x = y Then
                .BackColor = RGB(223, 243, 249)
            Else
                .BackColor = RGB(71, 142, 185)
            End If
        End With
             
    Next i
    Exit Sub
SetValue:
    Err.Clear
    x = 0
    y = 0
    Resume Next
End Sub