Fix ServiceExt::handle_error footgun (#120)

As described in
https://github.com/tokio-rs/axum/pull/108#issuecomment-892811637, a
`HandleError` created from `axum::ServiceExt::handle_error` should _not_
implement `RoutingDsl` as that leads to confusing routing behavior.

The technique used here of adding another type parameter to
`HandleError` isn't very clean, I think. But the alternative is
duplicating `HandleError` and having two versions, which I think is less
desirable.

src/handler/mod.rs

@@ -5,7 +5,7 @@ use crate::{
     extract::FromRequest,
     response::IntoResponse,
     routing::{future::RouteFuture, EmptyRouter, MethodFilter},
-    service::HandleError,
+    service::{HandleError, HandleErrorFromRouter},
 };
 use async_trait::async_trait;
 use bytes::Bytes;
@@ -371,7 +371,7 @@ impl<S, T> Layered<S, T> {
     pub fn handle_error<F, ReqBody, ResBody, Res, E>(
         self,
         f: F,
-    ) -> Layered<HandleError<S, F, ReqBody>, T>
+    ) -> Layered<HandleError<S, F, ReqBody, HandleErrorFromRouter>, T>
     where
         S: Service<Request<ReqBody>, Response = Response<ResBody>>,
         F: FnOnce(S::Error) -> Result<Res, E>,