Handle unsupported docker image types better

[tas50]

Describe the bug
If the go-containerregistry library can't handle an old manifest it throws the user a warning with a link to their repo. We should catch these and throw our own warning.

cnspec scan docker image nginx:1.7.11 --config ~/Downloads/edge-default.json
→ loaded configuration from /Users/tsmith/Downloads/edge-default.json using source --config
→ using service account credentials
→ discover related assets for 1 asset(s)
x unable to create runtime for asset error="rpc error: code = Unknown desc = unsupported MediaType: \"application/vnd.docker.distribution.manifest.v1+prettyjws\", see https://github.com/google/go-containerregistry/issues/377" asset=
Asset: ()
----------

error: rpc error: code = Unknown desc = unsupported MediaType: "application/vnd.docker.distribution.manifest.v1+prettyjws", see https://github.com/google/go-containerregistry/issues/377


Scanned 1 asset