[gcs] Don't crash if the GCP response does not contain an error_uri

Sometimes the GCS API error response does not contain
"error_description" or "error_uri" fields. If we make these fields
required, then it makes it hard to actually debug issues since it fails
due to a json unwrapping issue.

This CL makes those two fields optional.

Change-Id: If39db323b1a11e7816c34e33ff6ca8a765403e5f
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1512114
Fuchsia-Auto-Submit: Jerry Belton <jcecil@google.com>
Reviewed-by: Jiaming Li <lijiaming@google.com>
Commit-Queue: Jerry Belton <jcecil@google.com>