Gracefully handle missing "r8.im" in the image name

[bfirsh]

When pushing to Replicate, I can imagine a very common mis-step is missing "r8.im" in the image name. E.g. cog push bfirsh/resnet when I mean cog push r8.im/bfirsh/resnet.

This is Replicate specific, and we should probably avoid adding much Replicate specific code to Cog. But this is probably something where we can be totally compatible with Docker and still handle this case. E.g. if it 404s on Docker Hub, then check if it exists on Replicate, and if it does, give you a hint that you should prefix the image with r8.im/.