Improve error message when no resources have been added to operator

[jroper]

If you create a new operator, and then run operator-sdk generate k8s without first creating any custom resources, as the getting started guide was instructing people to do until operator-framework/getting-started#30 fixed it, you get this less than helpful error message:

$ operator-sdk generate k8s
Running code-generation for custom resource group versions: []
2018/11/12 14:32:03 failed to perform code-generation: (exit status 255)

This could be improved to something more informative so that if people do do this (for example, if they accidentally miss the step that I added to the getting started guide), they have some hope of working out what they did wrong.

[hasbro17]

We forgot to print out the combined output (stdout and stderr) when the command fails. Right now we're only doing it after it succeeds. We need to make the following change:

log.Fatalf("failed to perform code-generation: (%v) %v", err, out)

https://github.com/operator-framework/operator-sdk/blob/master/commands/operator-sdk/cmd/generate/k8s.go#L78

[estroz]

#707 sends errors to Stderr.