Description of problem:

Requesting blob from Pulp that doesn't exist results in 500 

How reproducible:

100%

Is this issue a regression from an earlier version:

First time trying

Steps to Reproduce:

1. GET a blob that doesn't exist via the Pulp registry. This can be tested via a `podman push` on the container gateway for a repo that exists but with a new image.2.

Actual behavior:

A 500 is returned with a backtrace

Expected behavior:
A proper error should get returned that follows the OCI registry spec.

Business Impact / Additional info: