`pkgdev manifest` : should not generate manifest (or keep files) in the event of a failed download

[Kangie]

I've been experiencing some issues with HTTP/2 streams from GitHub recently.

Each time a download fails, I need to remove the manifest and downloaded files, even though there was clearly an issue and I can't imagine curl returning an exit code of 0, yet:

• A definitely broken file is maintained in ${DISTFILES}
• a manifest is still generated.

kangie@monolith /var/db/repos/gentoo/www-client/chromium (master?) $ pkgdev manifest        
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
 60 1532M   60  930M    0     0  3160k      0  0:08:16  0:05:01  0:03:15 2713k
curl: (92) HTTP/2 stream 1 was not closed cleanly: PROTOCOL_ERROR (err 1)
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
 86  882M   86  760M    0     0  2458k      0  0:06:07  0:05:16  0:00:51  526k
curl: (92) HTTP/2 stream 1 was not closed cleanly: PROTOCOL_ERROR (err 1)
 * generating manifest: www-client/chromium::gentoo

See also: #153 #197