fix: RouteNotFound handler does not falls back to root one

[majiayu000]

Fixes #2485

Changes

• Store RouteNotFound handler for "/*" path on Echo instance for fallback use
• Modify group middleware to check for root RouteNotFound handler before using default
• Add tests verifying root 404 handler fallback behavior for groups with middleware

[aldas]

Hi, happy new year!

This is interesting idea but I think you can achieve same by replacing default NotFoundHandler (assuming you do not deal with multiple echo instances which routes/groups are added in mixed order)

	echo.NotFoundHandler = func(c echo.Context) error {
		return echo.NewHTTPError(http.StatusNotFound, "custom route not found")
	}
	e.RouteNotFound("*", echo.NotFoundHandler)

	g := e.Group("/group") // <--- will use replaced echo.NotFoundHandler 

the root problem/cause here is that we have these 404 handlers added.

echo/group.go

Lines 27 to 33 in d0f9d1e

	// group level middlewares are different from Echo `Pre` and `Use` middlewares (those are global). Group level middlewares
	// are only executed if they are added to the Router with route.
	// So we register catch all route (404 is a safe way to emulate route match) for this group and now during routing the
	// Router would find route to match our request path and therefore guarantee the middleware(s) will get executed.
	g.RouteNotFound("", NotFoundHandler)
	g.RouteNotFound("/*", NotFoundHandler)
	}

Which is something we can not change but if we are looking for workaround I think replacing the 404handler is probably the easiest way.

[aldas]

also these group level 404 routes can be replaced

	my404 := func(c echo.Context) error {
		return echo.NewHTTPError(http.StatusNotFound, "custom route not found")
	}
	g := e.Group("/group")
	g.RouteNotFound("", my404)
	g.RouteNotFound("/*", my404)
	
	echo.NotFoundHandler = my404
	e.RouteNotFound("*", my404)