Improve function GetStateRangeScanIterator

- Close iterators if newItrCombiner fails.
- Handle nil iterators returned without errors.
- Improve error messages for debugging.

Signed-off-by: David Enyeart <enyeart@us.ibm.com>

Author: denyeart

core/ledger/kvledger/txmgmt/queryutil/query_executer_combiner.go

@@ -12,6 +12,7 @@ import (
 	"github.com/hyperledger/fabric/core/ledger/kvledger/txmgmt/privacyenabledstate"
 	"github.com/hyperledger/fabric/core/ledger/kvledger/txmgmt/statedb"
 	"github.com/hyperledger/fabric/core/ledger/util"
+	"github.com/pkg/errors"
 )
 
 var logger = flogging.MustGetLogger("util")
@@ -62,12 +63,21 @@ func (c *QECombiner) GetStateRangeScanIterator(namespace string, startKey string
 			for _, itr := range itrs {
 				itr.Close()
 			}
-			return nil, err
+			return nil, errors.Wrap(err, "failed to get iterator from QueryExecuter")
+		}
+		if itr == nil {
+			for _, itr := range itrs {
+				itr.Close()
+			}
+			return nil, errors.New("received nil iterator from QueryExecuter")
 		}
 		itrs = append(itrs, itr)
 	}
 	itrCombiner, err := newItrCombiner(namespace, itrs)
 	if err != nil {
+		for _, itr := range itrs {
+			itr.Close()
+		}
 		return nil, err
 	}
 	return itrCombiner, nil