Managing DHCP leases is only possible when the DHCP server is enabled

[DL6ER]

What does this implement/fix?

This PR adds error-handling for DHCP lease deletion presenting human-readable error messages on failures instead of always silent acceptance of the command (even if nothing is done).

Related issue or feature (if applicable): N/A

Pull request in docs with documentation (if applicable): N/A

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• The code change is tested and works locally.
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[github-actions]

This pull request has conflicts, please resolve those before we can evaluate the pull request.

[github-actions]

Conflicts have been resolved.

[yubiuser]

Looks good from a code perspective. I don't have a way to test this,