`containertool` should print better error messages for authentication failures

When authentication fails, `containertool` prints the 403 error without any further information:
Error: Registry returned an unexpected HTTP error code: 403

Start by reproducing an authentication failure with `containertool` and trace where the registry's 403 response is handled. Done means the command reports useful authentication-related information instead of only the unexpected HTTP status.

Written by the indexing model from the issue text.