k8swatch: simplify manifest subscriber logic

[milas]

While debugging, I cleaned this up a bit to make it easier to follow:

• Fatally error if the create fails - this should never happen, but it was being silently ignored before and would have meant that no K8s resources would be discovered for a resource silently if hit, so erroring out is the best course of action for now
• Removed redundant delete case (this made sense at one point but was an impossible to hit branch at this point)
• Used a value instead of a pointer when processing specs (was never actually nil + makes the diff logic easier to follow)