Add error to return type of extractor

[David]

Extractors may (gracefully) fail. In these cases, they should probably return an error type.
If they don't return an error, we have two options:
(1) They return an empty object: In this case, it seems like the data has been processed, which would be false
(2) The extractor panics. In this case another set of extractors will retry extracting the initial data, in this case we will be busy for no reason.