Improve messaging when trying to scan an unsupported manifest

Why is this change being made?

When failing to scan an image in manifest v1 format, container scanning produced this error, which is difficult to debug:

2022-11-04T20:16:49.532Z	FATAL	image scan error:
    github.com/aquasecurity/trivy/pkg/commands/artifact.Run
        /home/runner/work/trivy/trivy/pkg/commands/artifact/run.go:370
  - scan error:
    github.com/aquasecurity/trivy/pkg/commands/artifact.(*runner).scanArtifact
        /home/runner/work/trivy/trivy/pkg/commands/artifact/run.go:230
  - unable to initialize a scanner:
    github.com/aquasecurity/trivy/pkg/commands/artifact.scan
        /home/runner/work/trivy/trivy/pkg/commands/artifact/run.go:538
  - unable to initialize a docker scanner:
    github.com/aquasecurity/trivy/pkg/commands/artifact.imageStandaloneScanner
        /home/runner/work/trivy/trivy/pkg/commands/artifact/scanner.go:22
  - 4 errors occurred:
	* unable to inspect the image (registry.hub.docker.com/library/httpd:2.4.12): Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?
	* unable to initialize Podman client: no podman socket found: stat podman/podman.sock: no such file or directory
	* containerd socket not found: /run/containerd/containerd.sock
	* unsupported MediaType: "application/vnd.docker.distribution.manifest.v1+prettyjws", see https://github.com/google/go-containerregistry/issues/377

This MR updates our error mapping in improve_stderr_msg so that it will now output a more helpful message for this error condition.

Test job: https://gitlab.com/gitlab-org/govern/demos/container-scanning-test/-/jobs/3284417744