Validate credentials before performing the move

[migmartri]

Currently, if the target registry credentials are wrong, the system does not fail at first but instead tells you that pushing the images is required https://github.com/vmware-tanzu/asset-relocation-tool-for-kubernetes/blob/main/internal/container_registry.go#L47

Ideally we should be able to detect 401/403 errors and return here. We should double check that a similar validation is made for the source container repository too