notify error should invoke a separate client error handler

[plastikfan]

consider this:

func (a *navigationAgent) notify(params *agentNotifyParams) (SkipTraversal, error) {
	skip := SkipNoneTraversalEn

	if params.readErr != nil {
		if a.doInvoke.Get() {
			clone := params.current.clone()
			clone.Error = i18n.NewThirdPartyErr(params.readErr)

			// Second call, to report ReadDir error
			//
			if le := params.frame.proxy(clone, nil); le != nil {
				if errors.Is(params.readErr, fs.SkipAll) && (clone.IsDirectory()) {
					params.readErr = nil
				}

				return SkipAllTraversalEn, i18n.NewThirdPartyErr(params.readErr)
			}
		} else {
			return SkipAllTraversalEn, i18n.NewThirdPartyErr(params.readErr)
		}
	}

	return skip, nil
}

If an error occurs during the navigation process, the navigator will invoke the client callback with the item on the traverse item being set. Perhaps this is not the best way to go, becuase the client has to know that they are being invoked for a second time for the same traversal item but for an error. It would probably be better for the client to be able to specify an error callback specifically for the purpose of error handling, that way the prmary client callback can be kept clean and not have to be concerned about error handling.