http-lib: log reason that causes lack of response

Otherwise users are nonthewiser when a long migration fails. With this change
and a reproduction we can have the chance to understand the issue and maybe fix
it.

Backport of
f2d3a6ae3f and eb8d973c3c

Signed-off-by: Pau Ruiz Safont <pau.ruizsafont@cloud.com>

http-svr/http_client.ml

@@ -181,6 +181,7 @@ let response_of_fd_exn fd =
 
 (** [response_of_fd fd] returns an optional Http.Response.t record *)
 let response_of_fd ?(use_fastpath = false) fd =
+  let __FUN = "Http_client.response_of_fd" in
   try
     if use_fastpath then
       Some (response_of_fd_exn fd)
@@ -189,7 +190,12 @@ let response_of_fd ?(use_fastpath = false) fd =
   with
   | Unix.Unix_error (_, _, _) as e ->
       raise e
-  | _ ->
+  | e ->
+      Backtrace.is_important e ;
+      let bt = Backtrace.get e in
+      Debug.log_backtrace e bt ;
+      D.debug "%s: returning no response because of the exception: %s" __FUN
+        (Printexc.to_string e) ;
       None
 
 (** See perftest/tests.ml *)