Returning ExtraneousLocale in BlobDataProvider

[robertbastian]

And not in datagen

Fixes #3505

[sffc]

Technically speaking, both BlobDataProvider and DatagenProvider should be returning the ExtraneousLocale error. We're not doing it consistently everywhere in DatagenProvider. But I also understand that it's hard to enforce consistently across all the various impls of DatagenProvider.

[Manishearth]

In favor of the blob changes, feel iffy about the datagen ones; I don't mind having those around.

[robertbastian]

I removed the datagen ones because only a handful out of dozens of singleton implementations correctly return ExtraneousLocale. Currently the datagen provider's error cases aren't great across the board, for example if you call a non-singleton load with a locale that's not in supported_locales, you'll often get an IO error instead of a MissingLocale error. I'm happy to fix that separately if we feel like that's an issue.

[Manishearth]

seems fair. would like the defence in depth of having these errors in more places, but not particularly insistent