The `Image` resource already ignores errors during read but the `Index`
resource was requiring the inspect call to always succeed. This will often
fail due to https://github.com/pulumi/pulumi/issues/4981 (note however in
this case credentials are stored with the resource instead of the provider).
This changes our logic to instead emit a warning if the credentials are
invalid.
Fixes https://github.com/pulumi/pulumi-docker-build/issues/121.
