Test ftl.pi-hole.net availability

[DL6ER]

What does this implement/fix?

Add checking for availability of https://ftl.pi-hole.net when using FTL from a custom branch. If the server is down (or the user is offline, cannot resolve the domain, etc.), we fail early and hard instead of possibly corrupting the installation.

Note

This can easily be tested by checking out this branch and - thereafter - defining a local DNS record like

0.0.0.0 ftl.pi-hole.net

Then, pihole -up sees a situation where we have simulated a non-reachable FTL binary server:

---

Related issue or feature (if applicable): #5561 #5562

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]

This line (return 2) will never be executed.

When the new if-then-else command above is executed, one of the return commands will be executed before.

(Suggestion button is not available for this line)

[DL6ER]

Thanks, fixed