Improve reporting of error states

Distinguish between "the definition of the header row(s) is bad"
and "the current Selection is invalid" error states in
the reporting into the status box on the form.

src/UFExporter.frm

@@ -43,6 +43,7 @@ Attribute appn.VB_VarHelpID = -1
 Const NoFolderStr As String = "<none>"
 Const InvalidSelStr As String = "<invalid selection>"
 Const NoHeaderRngStr As String = "<no header>"
+Const BadHeaderDefStr As String = "<invalid definition>"
 
 
 ' =====  GLOBALS  =====
@@ -444,15 +445,28 @@ Private Function getHeaderRangeAddress() As String
 
     Dim headerRange As Range
 
+    ' Store the return value from retrieving the header range
     Set headerRange = getHeaderRange
 
     If ChBxHeaderRows.Value Then
-        If Not headerRange Is Nothing And checkHeaderRowValues Then
+        ' Header range has to be defined in order for there to be
+        ' an address to return. The validity of the header row
+        ' definition in the userform is already checked
+        ' within getHeaderRange, and so it doesn't need(?) to be
+        ' checked again here.
+        If Not headerRange Is Nothing Then
             getHeaderRangeAddress = getHeaderRange.Address( _
                         RowAbsolute:=False, ColumnAbsolute:=False _
             )
         Else
-            getHeaderRangeAddress = InvalidSelStr
+            ' Though, it's clearer to change the error message in the display box
+            ' depending on whether the header definition is invalid,
+            ' or if the actual range selection on ActiveSheet is bad
+            If Not checkHeaderRowValues Then
+                getHeaderRangeAddress = BadHeaderDefStr
+            Else
+                getHeaderRangeAddress = InvalidSelStr
+            End If
         End If
     Else
         getHeaderRangeAddress = NoHeaderRngStr