Panic when attempting to add a route_layer to an empty router

[jplatte]

Maybe we should also disallow / heavily discourage routers that only consist of a fallback and panic in the layer methods as well?

[davidpdrsn]

Makes sense! I've definitely heard some people who were tripped by this.

I think Router::new().layer(...) should be allowed, even though its a very roundabout way of handling all requests and adding a middleware to that.