Support Content Negotiation with @ExceptionHandler

Prior to this commit, `@ExceptionHandler` annotated controller methods
could be mapped using the exception type declaration as an annotation
attribute, or as a method parameter.
While such methods support a wide variety of method arguments and return
types, it was not possible to declare the same exception type on
different methods (in the same controller/controller advice).

This commit adds a new `produces` attribute on `@ExceptionHandler`; with
that, applications can vary the HTTP response depending on the exception
type and the requested content-type by the client:

```
@ExceptionHandler(produces = "application/json")
public ResponseEntity<ErrorMessage> handleJson(IllegalArgumentException exc) {
	return ResponseEntity.badRequest().body(new ErrorMessage(exc.getMessage(), 42));
}

@ExceptionHandler(produces = "text/html")
public String handle(IllegalArgumentException exc, Model model) {
	model.addAttribute("error", new ErrorMessage(exc.getMessage(), 42));
	return "errorView";
}
```

This commit implements support in both Spring MVC and Spring WebFlux.

Closes gh-31936

framework-docs/modules/ROOT/pages/web/webflux/controller/ann-exceptions.adoc

@@ -7,43 +7,8 @@
 `@ExceptionHandler` methods to handle exceptions from controller methods. The following
 example includes such a handler method:
 
-[tabs]
-======
-Java::
-+
-[source,java,indent=0,subs="verbatim,quotes",role="primary"]
-----
-	@Controller
-	public class SimpleController {
-
-		// ...
-
-		@ExceptionHandler // <1>
-		public ResponseEntity<String> handle(IOException ex) {
-			// ...
-		}
-	}
-----
-<1> Declaring an `@ExceptionHandler`.
-
-Kotlin::
-+
-[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
-----
-	@Controller
-	class SimpleController {
-
-		// ...
-
-		@ExceptionHandler // <1>
-		fun handle(ex: IOException): ResponseEntity<String> {
-			// ...
-		}
-	}
-----
-<1> Declaring an `@ExceptionHandler`.
-======
 
+include-code::./SimpleController[indent=0]
 
 
 The exception can match against a top-level exception being propagated (that is, a direct
@@ -65,6 +30,22 @@ Support for `@ExceptionHandler` methods in Spring WebFlux is provided by the
 `HandlerAdapter` for `@RequestMapping` methods. See xref:web/webflux/dispatcher-handler.adoc[`DispatcherHandler`]
 for more detail.
 
+[[webflux-ann-exceptionhandler-media]]
+== Media Type Mapping
+[.small]#xref:web/webmvc/mvc-controller/ann-exceptionhandler.adoc#mvc-ann-exceptionhandler-media[See equivalent in the Servlet stack]#
+
+In addition to exception types, `@ExceptionHandler` methods can also declare producible media types.
+This allows to refine error responses depending on the media types requested by HTTP clients, typically in the "Accept" HTTP request header.
+
+Applications can declare producible media types directly on annotations, for the same exception type:
+
+
+include-code::./MediaTypeController[tag=mediatype,indent=0]
+
+Here, methods handle the same exception type but will not be rejected as duplicates.
+Instead, API clients requesting "application/json" will receive a JSON error, and browsers will get an HTML error view.
+Each `@ExceptionHandler` annotation can declare several producible media types,
+the content negotiation during the error handling phase will decide which content type will be used.
 
 
 [[webflux-ann-exceptionhandler-args]]