Add (a somewhat hacked) future format error for page layout files.

common/page_layout/page_layout_reader.cpp

@@ -128,6 +128,14 @@ void PAGE_LAYOUT_READER_PARSER::Parse( WORKSHEET_LAYOUT* aLayout )
         if( token == T_LEFT )
             token = NextTok();
 
+        if( token == T_kicad_wks || token == T_drawing_sheet )
+        {
+            THROW_PARSE_ERROR( _( "KiCad was unable to open this file because it was created with "
+                                  "a more recent version than the one you are running.\n\n"
+                                  "To open it you will need to upgrade KiCad to 5.99 or later." ),
+                               CurSource(), CurLine(), CurLineNumber(), CurOffset() );
+        }
+
         if( token == T_page_layout )
             continue;
 

common/page_layout/page_layout_reader.keywords

@@ -1,3 +1,5 @@
+drawing_sheet
+kicad_wks
 page_layout
 setup
 left_margin