Added useful error message when image not found

[honeyankit]

Context

When an image is not found in the private registry, it throws an exception of DockerRegistry2::NotFound and lot of stack trace but this info is not useful for the end user.

updater | ERROR <job_500122728> Error processing myreg/ubuntu (DockerRegistry2::NotFound)
updater | ERROR <job_500122728> 404 Not Found
updater | ERROR <job_500122728> /home/dependabot/dependabot-updater/vendor/ruby/3.1.0/gems/docker_registry2-1.12.0/lib/registry/registry.rb:285:in `rescue in doreq'
updater | ERROR <job_500122728> /home/dependabot/dependabot-updater/vendor/ruby/3.1.0/gems/docker_registry2-1.12.0/lib/registry/registry.rb:269:in `doreq'
updater | ERROR <job_500122728> /home/dependabot/dependabot-updater/vendor/ruby/3.1.0/gems/docker_registry2-1.12.0/lib/registry/registry.rb:26:in `doget'

Error logs: https://github.com/dsp-testing/docker-firewalled-testing/network/updates/500122728

Approach

I'm using a rescue block to catch the "DockerRegistry2::NotFound" exception and throwing it with useful error message when a Dependabot tries to retrieve an image from the registry but it doesn't exist there.

[deivid-rodriguez]

Thanks for improving error messages! I just added two minor comments.

[jeffwidman]

IMO let's hold off merging this for a couple of days to see if we can land a fix in upstream, as it should be straightforward and that'd be less work than merging this then backing it out when upstream lands a fix. I pinged the maintainer on @honeyankit 's issue, hopefully we get a quick response.

[jeffwidman]

Pending fix upstream: deitch/docker_registry2#68

[deivid-rodriguez]

For what it's worth, I'm looking into actually deleting all of this code at #6150 since I believe it's problematic.

[jeffwidman]

IMO we should not merge this because even if if the underlying code doesn't get deleted in #6150, the upstream lib will still get patched in deitch/docker_registry2#68.

So not worth the trouble to add it here, then pulling it back out once upstream fix arrives...

[honeyankit]

This PR is not relevant as the fix is coming from upstream: deitch/docker_registry2#68.
So closing this PR.