Fixes issue

Describe the change

The new output makes it more apparent when the base image has changed to casual users. Tests were added, which required some work to pass through regclient.Opts. The empty error message is also suppressed to avoid an extra linefeed to stderr.

How to verify it

# report if base image has changed using annotations
regctl image check-base ghcr.io/regclient/regctl:alpine

# suppress the normal output with --quiet for scripts
if ! regctl image check-base ghcr.io/regclient/regctl:alpine --quiet; then
  echo build a new image here
fi

Changelog text

• Feat: Improve regctl image check-base output.

Please verify and check that the pull request fulfills the following requirements

• Tests have been added or not applicable
• Documentation has been added, updated, or not applicable
• Changes have been rebased to main
• Multiple commits to the same code have been squashed