sbom: don't error out when HTTP request returns error

Sometimes http request errors out without returning a
return code, which is not expected and causes SBOM
generation failure. We should treat the error same as
target URI doesn't exist.

BUG=b/323569827
TEST=presubmit
RELESAE_NOTE=None

Change-Id: Idb48d904f500ed5a059a71050406d73e5d75264b
Reviewed-on: https://cos-review.googlesource.com/c/third_party/platform/crosutils/+/64613
Reviewed-by: Sejal Sharma <sejalsharma@google.com>
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>