context.TODO（）.Err（）将始终返回nil，这在the source code中很容易看到：

package context

// An emptyCtx is never canceled, has no values, and has no deadline.
type emptyCtx int

func (*emptyCtx) Err() error {
    return nil
}

// ...

var (
    todo       = new(emptyCtx)
)

// ...

// TODO returns a non-nil, empty Context. Code should use context.TODO when
// it's unclear which Context to use or it is not yet available (because the
// surrounding function has not yet been extended to accept a Context
// parameter).
func TODO() Context {
    return todo
}