Errors when loading the configuration file should be easier to read

[matrixbot]

This issue was originally created by @sandhose at matrix-org/matrix-authentication-service#2491.

Because we use figment to handle configuration, we sometimes have really weird error messages. See #2477 for example.

A few things we should do:

• avoid #[serde(flatten)], as it breaks error reporting
• tag important places of the config so that we can
• do configuration post-processing instead
• actually test some misconfiguration scenarios