users/gcp: convert to new errors package
users/gcp: convert to new errors package

Change-Id: I9b458b8162b71fad58d77218120d1558f4ebb2a5
Reviewed-on: https://upspin-review.git.corp.google.com/2332
Reviewed-by: David Presotto <516b9783fca517eecbd1d064da2d165310b19759@google.com>
Reviewed-by: Eduardo Pinheiro <55040cbb39ba395339ae7940d8da6cd97797a4a8@google.com>

upspin/upspin,upspin/upspin