Improve error message if the user is not logged in

Previously the error message prompted the user to `docker login`, but
this will not work if
- 2FA is required
- SSO is enabled with enfocement on Docker Hub,
  see https://docs.docker.com/single-sign-on/#enforce-sso-in-docker-hub

The most reliable way to login is through the Docker Desktop UI, since
the SSO/2FA flows are all handled automatically (via the web browser)

Try to improve the error message by explaining the options.

Signed-off-by: David Scott <dave@recoil.org>

Author: djs55

internal/provider/provider.go

@@ -187,7 +187,14 @@ func WithExperimental() Ops {
 func getToken(opts Options) (string, error) {
 	if opts.auth.Username == "" {
 		return "", fmt.Errorf(`You need to be logged in to Docker Hub to use the scan feature.
-please login to Docker Hub using the Docker Login command`)
+
+If you are not using Docker Desktop, either
+- use the "docker login" CLI command with a username and password. Note this will not work if
+  2FA is required or if SSO enforcement is enabled on Docker Hub; or
+- use the "docker login" CLI command with a username and Personal Access Token. This requires
+  a token to be generated in advance.
+
+If you are using Docker Desktop: login via the UI or whale menu`)
 	}
 	h := hub.GetInstance()
 	jwks, err := h.FetchJwks()