auth, etcdserver: forbid invalid auth management

If auth is enabled,
1. deleting the user root
2. revoking the role root from the user root
must not be allowed. This commit forbids them.

etcdserver/api/v3rpc/util.go

@@ -97,6 +97,8 @@ func togRPCError(err error) error {
 		return rpctypes.ErrGRPCAuthNotEnabled
 	case auth.ErrInvalidAuthToken:
 		return rpctypes.ErrGRPCInvalidAuthToken
+	case auth.ErrInvalidAuthMgmt:
+		return rpctypes.ErrGRPCInvalidAuthMgmt
 	default:
 		return grpc.Errorf(codes.Unknown, err.Error())
 	}