Verify remote FTL checksum

[DL6ER]

What does this implement/fix?

Verify that we actually downloaded a valid checksum before comparing it to the local one. This covers situations where downloading the checksum from remote might have failed.

Before, we simply always believed that curl "will surely have worked" and might falsely compare an empty remote checksum to the local one always resulting in a checksum mismatch + FTL re-downloading. In case of an unreadable upstream checksum, we still re-download (seems the best compromise) but print a clear message telling the user what is going on.

This PR does not introduce any functional changes.

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Related issue or feature (if applicable): See Discourse thread linked below

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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[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/ftl-update-checksum-mismatch-beta-v6/68777/15