FAB-2357 join failed looking up ledger

https://jira.hyperledger.org/browse/FAB-2357

Not all proposals are on channels (and have a ledger). Some proposals
such as to the CSCC for join is "channelless".
In this instance, the failure is due to attempting to access the ledger
for doing TX uniqueness checks. Given there's no ledger, we have to
bypass ledger based uniqueness checks.

Change-Id: I3dbdf49307989575494efc15f3586bd4a85c41cb
Signed-off-by: Srinivasan Muralidharan <muralisr@us.ibm.com>

core/endorser/endorser.go

@@ -296,12 +296,16 @@ func (e *Endorser) ProcessProposal(ctx context.Context, signedProp *pb.SignedPro
 		return &pb.ProposalResponse{Response: &pb.Response{Status: 500, Message: err.Error()}}, err
 	}
 
-	lgr := peer.GetLedger(chainID)
-	if lgr == nil {
-		return nil, errors.New("Failure while looking up the ledger")
-	}
-	if _, err := lgr.GetTransactionByID(txid); err == nil {
-		return nil, fmt.Errorf("Duplicate transaction found [%s]. Creator [%x]. [%s]", txid, hdr.SignatureHeader.Creator, err)
+	//chainless proposals do not/cannot affect ledger and cannot be submitted as transactions
+	//ignore uniqueness checks
+	if !ischainless {
+		lgr := peer.GetLedger(chainID)
+		if lgr == nil {
+			return nil, errors.New(fmt.Sprintf("Failure while looking up the ledger %s", chainID))
+		}
+		if _, err := lgr.GetTransactionByID(txid); err == nil {
+			return nil, fmt.Errorf("Duplicate transaction found [%s]. Creator [%x]. [%s]", txid, hdr.SignatureHeader.Creator, err)
+		}
 	}
 
 	// obtaining once the tx simulator for this proposal. This will be nil