Warn the user when Zebra doesn't recognize the format of zebrad.toml

[upbqdn]

Motivation

When the format of zebrad.toml changes, Zebra doesn't recognize it appropriately and returns an unclear error saying that it couldn't read some data. This becomes an issue when users install a new version of Zebra that uses a new format of zebrad.toml, but keep their old zebrad.toml file.

Suggested solution

Zebra should provide a message saying that it didn't recognize some items in the zebrad.toml file, and suggest that the file might need to be updated.

Related Work

This issue depends on issue #4625.