No clear error message when scanning Docker container

[vdespa]

I've found three situations in which the following message is provided when running osv-scanner --docker someimage:

Scanned docker image with 0 packages
No package sources found, --help for usage information.

These situations are:

• the image name is incorrect
• the image is in a private registry that requires authentication
• the Docker client is installed, but the Docker daemon is not running

I would have expected an unambiguous error message to help with troubleshooting.