(TK-300) schema tweaks based on PR feedback
(TK-300) schema tweaks based on PR feedback

* Removed the schema for the return type from `run-lifecycle-fn!`
  because we don't use the return value anywhere.
* Added some additional assertions in the shutdown-on-error* fn,
  to bring us back to parity with our previous assertions.

puppetlabs/trapperkeeper,camlow325/trapperkeeper