BlobDataProvider should return DataErrorKind::ExtraneousLocale for singleton structs

[robertbastian]

Currently it returns MissingLocale, which is not correct, as it will make fallback continue. For example requesting de-DE for a singleton with fallback will try de-DE, de, und and actually succeed, where it should be an error because trying de-DE for such a key is a bad request.

[robertbastian]

Fixed in #3494 for databake.

[robertbastian]

We might want to add the singleton-ness of a KeyedDataMarker to the key metadata.

[sffc]

We might want to add the singleton-ness of a KeyedDataMarker to the key metadata.

+1

[sffc]

Discussion:

• Good to add a singleton field to the DataKeyMetadata
• Maybe add a load_singleton or load_und function to DataProvider (with a default implementation that calls load), but we're not convinced if it's actually useful
• We could make the singleton part of the type system, but @sffc thinks that's too much machinery and it's fine as metadata

LGTM: @sffc @robertbastian