Add HTTPRequestError class
    
    Currently, HTTP requests that respond with status not in the 2xx
    range raise RemoteRetrieveError.  The exception includes no
    information about the response status.
    
    Add the 'HTTPRequestError' class which extends 'RemoteRequestError'
    with an attribute for the response status, and update the Dogtag
    RestClient to raise the new error.
    
    Part of: https://fedorahosted.org/freeipa/ticket/6260
    Part of: https://fedorahosted.org/freeipa/ticket/3473
    
    Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>