[WIP]: broker ErrorHandler option (#1296)

* broker ErrorHandler option

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

* rewrite Event interface, add error

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

* implement new interface

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

* change ErrorHandler func to broker.Handler

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

* fix

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

server/rpc_event.go

@@ -7,6 +7,7 @@ import (
 
 // event is a broker event we handle on the server transport
 type event struct {
+	err     error
 	message *broker.Message
 }
 
@@ -19,6 +20,10 @@ func (e *event) Message() *broker.Message {
 	return e.message
 }
 
+func (e *event) Error() error {
+	return e.err
+}
+
 func (e *event) Topic() string {
 	return e.message.Header["Micro-Topic"]
 }