When creating a directory, Writer.Create now returns a dummy
io.Writer that always returns an error on Write.

Fixes #24043

Change-Id: I7792f54440d45d22d0aa174cba5015ed5fab1c5c
Reviewed-on: https://go-review.googlesource.com/108615Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>