Errors when loading the configuration file should be easier to read

[sandhose]

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

[matrixbot]

For your information, this issue has been copied over to the Element fork of matrix-authentication-service: element-hq/matrix-authentication-service#2491