kojivmd: check for HTTP errors in getFile()
    
    kojivmd proxies files from Koji's topurl through the getFile() RPC.
    In some cases (misconfigurations), kojivmd can fail to download files
    from topurl.
    
    Prior to this change, if kojivmd failed to download a file (for example,
    a 404 error), it would silently cache the 404 error HTTP body contents
    and pass those on to verifyChecksum(). As a result, kojivmd would verify
    the checksum of an HTML error page, rather than checksuming the intended
    Koji archive file. In this scenario, it's difficult for administrators
    to diagnose why checksums are not matching.
    
    Check the HTTP response for errors before doing anything with the
    response contents. With this change, winbuild tasks will fail with an
    easier-to-understand "HTTP 404 not found" HTTPError, rather than a Koji
    BuildError about checksums.