storage/gcs: retry on transient errors.
storage/gcs: retry on transient errors.

Change-Id: I6ba2b3f3879e2ab0f221af3e0ac80cbec69b6f14
Reviewed-on: https://upspin-review.googlesource.com/7326
Reviewed-by: Eduardo Pinheiro <55040cbb39ba395339ae7940d8da6cd97797a4a8@google.com>

upspin/upspin,upspin/upspin