Make APKINDEX downloads returning 404 non-fatal (!1726)

Do not fail when an APKINDEX can not be downloaded, print a WARNING
instead. This matches, what apk does. Add a new allow_404 parameter to
pmb.helpers.http.download(), and use it in pmb.helpers.repo.update()
for downloading APKINDEX files. Cache the APKINDEX URLs that gave a 404
for the session, so we do not attempt to download these again.

This is needed for the new binary repository: the initial build is done
without existing APKINDEX files, so we must not fail in that case.

pmb/helpers/args.py

@@ -130,7 +130,7 @@ def add_shortcuts(args):
 def add_cache(args):
     """ Add a caching dict (caches parsing of files etc. for the current
         session) """
-    repo_update = {"offline_msg_shown": False}
+    repo_update = {"404": [], "offline_msg_shown": False}
     setattr(args, "cache", {"apkindex": {},
                             "apkbuild": {},
                             "apk_min_version_checked": [],