Cleaning up DataError

[robertbastian]

The variants are now:

• MarkerNotFound
  • This was MissingDataMarker, unclear if it was missing from the request or the data
• IdentifierNotFound
  • This was MissingLocale, unclear if it was missing from the request or the data, and needs to use identifier terminology
• NeedsLocale
  • The locale is not optional, this was unused
• InvalidRequest
  • This was ExtraneousLocale but I think it should cover other illegal requests as well
• Filtered
  • This was FilteredResource, but we're not using resource terminology anymore
• InconsistentData
• Downcast
  • This was MismatchedType, but is only returned from AnyPayload::downcast and DowncastingAnyProvider, so I think this is a better name. Also more symmetric with ...
• Deserialize
  • This was UnavailableBufferFormat or Custom before. Now all serde issues return this variant, i.e. DataPayload::deserialize and DeserializingBufferProvider
• Custom
• Io
• MissingSourceData
  • DatagenProvider is just one provider implementation, it can use Custom

[sffc]

ok