Added specific error types rather than generic error types

Just as the title says! This makes it easier if the caller of feed.FetchClient() wants to dig into the error type further. e.g.

```go
err = feed.FetchClient(feedUrl, client, nil)
if err != nil {
    switch err.(type) {
        case *xml.SyntaxError:
            // Do stuff
        case *url.Error:
            // Do stuff
        case *feeder.UnsupportedFeedError:
            // Do stuff
        case *feeder.MissingRssNodeError:
            // Do stuff
        default:
            // do stuff
    }
}
```
Hope this helps!

Cheers,
Ben