Add DHCP netmask field and nicely format API errors

[DL6ER]

What does this implement/fix?

This PR accompanies pi-hole/FTL#1731. It adds a new DHCP netmask field and furthermore improve error displaying for settings changes.

Examples:

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

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

---

By submitting this pull request, I confirm the following:

1. I have read and understood the contributors guide, as well as this entire template. I understand which branch to base my commits and Pull Requests against.
2. I have commented my proposed changes within the code.
3. I am willing to help maintain this change if there are issues with it later.
4. It is compatible with the EUPL 1.2 license
5. I have squashed any insignificant commits. (git rebase)

Checklist:

• The code change is tested and works locally.
• I based my code and PRs against the repositories developmental branch.
• I signed off all commits. Pi-hole enforces the DCO for all contributions
• I signed all my commits. Pi-hole requires signatures to verify authorship
• I have read the above and my PR is ready for review.

[rdwebdesign]

The API uses dhcp.start and dhcp.end. These property names will be shown in docs and error messages.

Should we change the field names to Start and End?

[sy-be]

Thank you! Looking forward to v6.0. Atm netmask field is incorrectly generated in /etc/dnsmasq.d/02-pihole-dhcp.conf - it doesn't add netmask and inferred value isn't always correct.

[DL6ER]

@sy-bee Can you be a bit more specific about what is wrong and how the inferred value is not correct? It'd be great if you could open an issue in the FTL repository so we have a place where we can discuss things properly: https://github.com/pi-hole/FTL/issues/new

This PR added only displaying, the real work happens in FTL.

[sy-be]

Thanks @DL6ER , I have created an issue here with details: pi-hole/FTL#1947