feature: Log a better error message when a lock file for a different variant exists or make variant a required key

[edgarrmondragon]

Feature scope

CLI (options, error messages, logging, etc.)

Description

Currently, if the plugin variant is missing but there's a lock file for some variant, Meltano outputs a message like

Extractor 'tap-fleetio' is not known to Meltano. Try running `meltano lock --update --all` to ensure your plugins are up to date.

This is confusing to users because they're not directed to adding variant: fleetio to their plugin.