refactoring; move common validation into initialize()
refactoring; move common validation into initialize()

cyber-dojo/runner_stateless,cyber-dojo/runner,cyber-dojo/runner,cyber-dojo/runner_stateless,cyber-dojo/runner_stateless,cyber-dojo/runner_stateless,cyber-dojo/runner_stateless,cyber-dojo/runner_stateless,cyber-dojo/runner_stateless