Draft - Include more information on AlreadySentError errors

We have an app that is getting a lot of AlreadySentErrors. Likely
somewhere we have missed an `halt()`, but tracking that down is tricky.
I wonder if we could supply some more information in the various places
the error gets raised to help someone debug the issue?

Exact wording and contents can be discussed.

lib/phoenix/controller.ex

@@ -642,7 +642,9 @@ defmodule Phoenix.Controller do
     if state in @unsent do
       put_private_layout(conn, :phoenix_layout, :replace, layout)
     else
-      raise AlreadySentError
+      raise AlreadySentError, """
+      The response was already sent. Request was to `#{conn.request_path}` using #{inspect(layout)}\
+      """
     end
   end
 

test/phoenix/controller/controller_test.exs

@@ -89,7 +89,9 @@ defmodule Phoenix.Controller.ControllerTest do
     conn = put_format(conn, "print")
     assert layout(conn) == {AppView, :print}
 
-    assert_raise Plug.Conn.AlreadySentError, fn ->
+    message = "The response was already sent. Request was to `/` using {AppView, :print}"
+
+    assert_raise Plug.Conn.AlreadySentError, message, fn ->
       put_layout(sent_conn(), {AppView, :print})
     end
   end