Ensure documented behavior of returning ErrTxDone if the Tx has
already been committed or rolled back.

Fixes #18147

Change-Id: I07dc75bef4dbd4dd88dd252c96dc8ab99f28c00e
Reviewed-on: https://go-review.googlesource.com/33793
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>