v3rpc/rpctypes: use codes.FailedPrecondition for ErrGRPCNotLeader

Changes ErrGRPCNotLeader error code to FailedPrecondition,
to disable retry with unavailable.

Signed-off-by: Gyu-Ho Lee <gyuhox@gmail.com>

etcdserver/api/v3rpc/rpctypes/error.go

@@ -59,7 +59,7 @@ var (
 	ErrGRPCInvalidAuthMgmt      = status.New(codes.InvalidArgument, "etcdserver: invalid auth management").Err()
 
 	ErrGRPCNoLeader                   = status.New(codes.Unavailable, "etcdserver: no leader").Err()
-	ErrGRPCNotLeader                  = status.New(codes.Unavailable, "etcdserver: not leader").Err()
+	ErrGRPCNotLeader                  = status.New(codes.FailedPrecondition, "etcdserver: not leader").Err()
 	ErrGRPCNotCapable                 = status.New(codes.Unavailable, "etcdserver: not capable").Err()
 	ErrGRPCStopped                    = status.New(codes.Unavailable, "etcdserver: server stopped").Err()
 	ErrGRPCTimeout                    = status.New(codes.Unavailable, "etcdserver: request timed out").Err()