Add a fallback [`Handler`] to the router.
This service will be called if no routes matches the incoming request.
```rust
use axum::{
Router,
routing::get,
handler::Handler,
response::IntoResponse,
http::{StatusCode, Uri},
};
let app = Router::new()
.route("/foo", get(|| async { /* ... */ }))
.fallback(fallback);
async fn fallback(uri: Uri) -> (StatusCode, String) {
(StatusCode::NOT_FOUND, format!("No route for {}", uri))
}
# async {
# hyper::Server::bind(&"".parse().unwrap()).serve(app.into_make_service()).await.unwrap();
# };
```
Fallbacks only apply to routes that aren't matched by anything in the
router. If a handler is matched by a request but returns 404 the
fallback is not called.