Terminate http request on "416 Requested Range Not Satisfiable" response status code.

HTTP ERROR response 416 Requested Range Not Satisfiable [] This sometimes happens when a server responds incorrectly to a "continue" request. There is not enough information available (Range of the request) to properly handle this case. If it were, the case that the file has already been completely downloaded could be detected and an appropriate message could be generated. I also took the liberty to port the status codes from the old wget source to libwget/http.h to make the code more readable.