support/download: catch post-process errors

Fixes:
http://autobuild.buildroot.net/results/12a/12a63ae177fe3ed0c9a1ef2fa01870f334f36b0f/



Currently, when the post-process helper fails while downloading from
upstream, there is no fallback to the backup mirror.

In case the post-process helper fails, we must consider that to be a
download failure, so we must bail out as if the download backend itself
did fail, but we fail to do so.

Duplicate the logic we have for the download helper: if the post-process
helper fails, remove the downloaded stuff, and continue on to the next
URI, which will ultimately hit the backup mirror (if one has been
configured).

Reported-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit a39116b5)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>