debuginfo downloader: fix DebugInfoDownload::download() error paths.

One error path was returning nothing - the caller doesn't expect it.
Return RETURN_FAILURE instead.
Another error path was just exiting (and using non-standard logging):
converted to RETURN_FAILURE.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Signed-off-by: Jakub Filak <jfilak@redhat.com>