Improve error message on tag resolution failure

[halvards]

Digester currently uses HEAD requests to look up image digests.

If the image registry returns an error, there is no response body to provide additional information about the error.

As long as the HTTP status code was not 429, digester could fall back to a GET request in order to get the error message and surface it to the user.

[newtondev]

This has already been implemented in the gcrane package, please see https://github.com/google/go-containerregistry/blob/main/pkg/crane/digest.go#L45