[FAB-3252] improve event security check error msg

If the event message is not properly signed, the peer returns the
following error:

event message must be properly signed by an identity from a
participating organization in any of the channels existing in the
peer

This is not accurate because the latest logic is only check against
the local MSP, no checks against channel is done for v1.0.

Change-Id: I60f0971bc755565e2cd38d507c1433b52960f1d2
Signed-off-by: Jim Zhang <jzhang@us.ibm.com>

events/producer/handler.go

@@ -17,7 +17,6 @@ limitations under the License.
 package producer
 
 import (
-	"errors"
 	"fmt"
 	"strconv"
 
@@ -102,7 +101,7 @@ func (d *handler) deregisterAll() {
 func (d *handler) HandleMessage(msg *pb.SignedEvent) error {
 	evt, err := validateEventMessage(msg)
 	if err != nil {
-		return errors.New("event message must be properly signed by an identitiy from a participating organization in any of the channels existing in the peer")
+		return fmt.Errorf("event message must be properly signed by an identity from the same organization as the peer: [%s]", err)
 	}
 
 	switch evt.Event.(type) {