add ErrorHandlers middleware

guide/src/qs_10.md

@@ -210,3 +210,37 @@ fn main() {
 #     let _ = sys.run();
 }
 ```
+
+## Error handlers
+
+`ErrorHandlers` middleware allows to provide custom handlers for responses.
+
+You can use `ErrorHandlers::handler()` method  to register a custom error handler
+for specific status code. You can modify existing response or create completly new
+one. Error handler can return response immediately or return future that resolves
+to a response.
+
+```rust
+# extern crate actix_web;
+use actix_web::{
+    App, HttpRequest, HttpResponse, Result,
+    http, middleware::Response, middleware::ErrorHandlers};
+
+fn render_500<S>(_: &mut HttpRequest<S>, resp: HttpResponse) -> Result<Response> {
+   let mut builder = resp.into_builder();
+   builder.header(http::header::CONTENT_TYPE, "application/json");
+   Ok(Response::Done(builder.into()))
+}
+
+fn main() {
+    let app = App::new()
+        .middleware(
+            ErrorHandlers::new()
+                .handler(http::StatusCode::INTERNAL_SERVER_ERROR, render_500))
+        .resource("/test", |r| {
+             r.method(http::Method::GET).f(|_| HttpResponse::Ok());
+             r.method(http::Method::HEAD).f(|_| HttpResponse::MethodNotAllowed());
+        })
+        .finish();
+}
+```