cloud/storage/gcs: return NotExist on Delete, tidy up error handling
cloud/storage/gcs: return NotExist on Delete, tidy up error handling

The error handling in Delete didn't test for the API-specific "not
exist" error return, so code upstream wasn't seeing a NotExist error
Kind. Also add errors.Op in a few places where it was missing.

Change-Id: Ieb785db4045f676bbdba9d361b0f99cbe2776046
Reviewed-on: https://upspin-review.googlesource.com/17740
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/gcp,upspin/gcp