compress/flate: forward upstream Writer errors

compress/flate: forward upstream Writer errors

If the upstream writer has returned an error, it may not
be returned by subsequent calls.

This makes sure that if an error has been returned, the
Writer will keep returning an error on all subsequent calls,
and not silently "swallow" them.

Change-Id: I2c9f614df72e1f4786705bf94e119b66c62abe5e
Reviewed-on: https://go-review.googlesource.com/20515
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>