Fix potential leak of HTTP response body in downloadJSON

[marcusperlick]

1. According to standard library the client must close the Body of http response. The function downloadJSON() in the csaf_aggregator defers the Body.Close() after some checks are made and an early return that may return even when c.Get() does not return an error. This is where the res.Body might not be closed.
2. Besides that either I didn't get the idea behind the return via a call to a closure or it should be simplified.
3. Do not drop the err details from c.Get() but give them more context. See Go Blog: It is the error implementation’s responsibility to summarize the context.