better error message

weed/topology/node.go

@@ -168,7 +168,7 @@ func (n *NodeImpl) ReserveOneVolume(r int) (assignedNode *DataNode, err error) {
 			}
 		}
 	}
-	return
+	return nil, errors.New("No free volume slot found!")
 }
 
 func (n *NodeImpl) UpAdjustMaxVolumeCountDelta(maxVolumeCountDelta int) { //can be negative