CA-406770: Improve error message

[BengangY]

1. WLB request will raise wlb_authentication_failed when catching Http_client.Http_error. But actually only error code 401 and 403 should raise this type exception. For other error code, raise wlb_connection_reset. Also print the detail error code and message.
2. message_forwarding raises same error for Http_request_rejected and Connection_reset so we don't know which exception actually be raised. Print detailed logs for these 2 exceptions.

[psafont]

I think all three can be condensed into one:

Suggested change

	| Xmlrpc_client.Connection_reset | Http_client.Http_request_rejected _ ->
	warn
	"Caught Connection_reset when contacting host %s; converting into \
	| Xmlrpc_client.Connection_reset ->
	error
	"%s: Caught Connection_reset when contacting host %s; converting into \
	CANNOT_CONTACT_HOST"
	(Ref.string_of host) ;
	__FUNCTION__ (Ref.string_of host) ;
	raise
	(Api_errors.Server_error
	(Api_errors.cannot_contact_host, [Ref.string_of host])
	)
	| Http_client.Http_request_rejected s ->
	error
	"%s: Caught Http_request_rejected (%s) when contacting host %s; \
	converting into CANNOT_CONTACT_HOST"
	__FUNCTION__ s (Ref.string_of host) ;
	raise
	(Api_errors.Server_error
	(Api_errors.cannot_contact_host, [Ref.string_of host])
	)
	| Xmlrpc_client.Stunnel_connection_failed ->
	warn
	"Caught Stunnel_connection_failed while contacting host %s; converting \
	into CANNOT_CONTACT_HOST"
	(Ref.string_of host) ;
	error
	"%s: Caught Stunnel_connection_failed while contacting host %s; \
	converting into CANNOT_CONTACT_HOST"
	__FUNCTION__ (Ref.string_of host) ;
	| (Xmlrpc_client.Connection_reset | Http_client.Http_request_rejected _ | Xmlrpc_client.Stunnel_connection_failed) as e ->
	error
	"%s: Caught %s when contacting host %s; converting into \
	CANNOT_CONTACT_HOST"
	__FUNCTION__ (Printexc.to_string e) (Ref.string_of host)
	raise Api_errors.(Server_error (cannot_contact_host, [Ref.string_of host]))

[BengangY]

But I want to print the error string in Http_client.Http_request_rejected which may include an HTTP error code.

let http_rpc_recv_response use_fastpath error_msg fd =
  match response_of_fd ~use_fastpath fd with
  | None ->
      raise (Http_request_rejected error_msg)
  | Some response -> (
    match response.Http.Response.code with
    | ("401" | "403" | "500") as http_code ->
        raise (Http_error (http_code, error_msg))
    | "200" ->
        response
    | code ->
        raise (Http_request_rejected (Printf.sprintf "%s: %s" code error_msg))
  )

[psafont]

I would thinkPrintexc.to_string would include the error code as well.