This commit add a new DataError Variant for feeds that return 304.
Its expected behaviror and the current API of Source::into_feed
is kinda limiting the return type to make it easier to handle.

Up till now 304 was returning an Error to early return. Ideally
Source::into_feed will return a Multi variant Result Enum.

example:

enum FeedResult {
    Ok(Success(feed)),
    Ok(NotModified),
    Err(err),
}

Hopefully in a refactor in the near Future™

Till then we will just have to match and ignore
DataError::FeedNotModified.