Warn on unrecognised config section

[darkphoenix]

I've recently (apparently without realising) been updated to Mopidy 3.0, which, as I now know, doesn't include the MPD frontend in the core anymore and apparently didn't pull it in as a dependency, so I was left trying to troubleshoot why my Mopidy server was suddenly not listening on port 6600 anymore.
A feature that would've been very helpful would be a warning of some kind if there is a section in the configuration that has no corresponding extension - for example: "configuration for missing extension mpd - do you need to install mopidy-mpd?" (with the latter part left out for any configuration section that doesn't match the well-known first-party extensions, or rewritten in a more generic way).
As of now, the only sign you get the mpd extensions is gone is that it disappears from the listed extensions and enabled frontends, and I feel like a warning like that would be very helpful for future troubleshooting.

[kingosticks]

We currently emit debug messages reporting config sections that don't exist e.g

MainThread DEBUG    2020-02-02 23:42:31,718 Ignoring unknown config section: foo

Upgrading this to a warning sounds reasonable.

[jodal]

I guess the original thought behind this being only on debug level might be to cater for new "extensions" or hacks storing their configuration in mopidy.conf without there necessarily being a matching proper extension installed. I don't believe that to be an important use case, and logging a warning won't break the use case either.

I see advantages to increasing this to warning level:

• Typos in config section names are easier detected
• Config sections for uninstalled extensions are warned about so that the config file can be cleaned up.

I suggest the following wording: Ignoring config section {ext_name!r}: A matching extension was not found