MDL-81948 theme: fix fallback layout logic

Finding the page layout fallback was trying to get a default layout from
the ancient theme called "base". This default theme do not exists
anymore and defining a wrong layout in a theme is a code mistake that
should not be bypassed.

lib/outputlib.php

@@ -2499,9 +2499,7 @@ class theme_config {
             }
         }
 
-        debugging('Can not find layout file for: ' . $pagelayout);
-        // fallback to standard normal layout
-        return "$CFG->dirroot/theme/base/layout/general.php";
+        throw new coding_exception('Can not find layout file for: ' . $pagelayout . ' (' . $layoutfile . ')');
     }
 
     /**