It looks, that we sometimes hit a problem with incorrect downloads via this entry point. It is caused by malfunction of caches in the way or some network issues. Now, error is manifested a bit later when consistency of rpm file is checked. It introduces additional step in debugging process - checking if file is really broken or not (which mostly is not). Add better checks in openRemoteFile if file comes via http(s).

1. check http's Content-Length compared to what we really downloaded (requests ignores if it doesn't match, it makes no sense for koji
2. If file is rpm (other known type), additional info can be provided (e.g. checksum, or size fromb) - compare with checks in #2080 (maybe these can be reused here)
3. if errror is found, try to redownload (1-3 times with doubling delays)