[DI] Added safeguards against invalid config in the YamlFileLoader

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11333
| License       | MIT
| Doc PR        | n/a

Exceptions explaining the mistake are better than fatal errors or weird notices appearing when trying to deal with such invalid data.

The XML file loader is not affected by this because the data are validated with the XSD before being processed
Ah, I actually missed the `key` part. Still early in the morning. :wink: