Errors when YAML parsing fails are not intuitive

[hausdorff]

If you point k8s.yaml.ConfigFile at a URL that doesn't exist, but you still have transforms, you get an error like this:

error: Running program '/Users/erinkrengel/workspace/pulumi/gke-hello-world' failed with an unhandled exception:
    error: TypeError: Cannot read property 'labels' of undefined
        at k8s.yaml.ConfigFile.transformations (/Users/erinkrengel/workspace/pulumi/gke-hello-world/index.ts:116:30)
        at parseYamlObject (/Users/erinkrengel/workspace/pulumi/gke-hello-world/node_modules/@pulumi/kubernetes/provider.js:144:13)
        at parseYamlDocument (/Users/erinkrengel/workspace/pulumi/gke-hello-world/node_modules/@pulumi/kubernetes/provider.js:125:33)

This is confusing, since the error we should get is that the URL doesn't exist.

This happens in other places, too, so we should probably fix it.