code: private static boolean markError(OverlayContainer overlay, String errorTag) {
          if ( overlay.getFromNonPersistentCache(errorTag, WebExtAdapter.class) == null ) {
              overlay.addToNonPersistentCache(errorTag, WebExtAdapter.class, errorTag);
              return true;
          } else {
              return false;
          }
      }
docs: Mark an error condition to an overlay container.
  
  The tag is used as both the overlay key and the overlay value.
  
  Each mark is not placed relative to particular data. Each mark
  may be placed at most once.
  
  @param overlay The overlay in which to place the mark.
  @param errorTag The tag used as the mark.
  
  @return True or false telling if this is the first placement
  of the tag to the overlay.