Don't raise error for pushing existing image with existing digest

[geoffharcourt]

We bumped into an unusual issue today, where pushing an image twice to the same tag resulted in an error from AWS that blew up the Buildkite step. In this case we'd want to proceed, since the image we expect to see is in the proper place.

Here was the error as we saw it in Buildkite:

An error occurred (ImageAlreadyExistsException) when calling the PutImage operation: Image with digest 'sha256:<redacted>' and tag 'canary' already exists in the repository with name '<redacted>' in registry with id '<redacted>'