HDDS-1022. Add cmd type in getCommandResponse in SCMDatanodeProtocolServer. Contributed by Bharat Viswanadham.

hadoop-hdds/server-scm/src/main/java/org/apache/hadoop/hdds/scm/server/SCMDatanodeProtocolServer.java

@@ -342,7 +342,8 @@ public class SCMDatanodeProtocolServer implements
               ((ReplicateContainerCommand)cmd).getProto())
           .build();
     default:
-      throw new IllegalArgumentException("Not implemented");
+      throw new IllegalArgumentException("Scm command " +
+          cmd.getType().toString() + " is not implemented");
     }
   }