message: Fail with better error when thread not found
diff: @@ -574,6 +574,10 @@ export class BotkitConversation<O extends object = {}> extends Dialog<O> {
  // Let's interpret the current line of the script.
  const thread = this.script[step.thread];
  + if (!thread) {
  + throw new Error(`Thread '${step.thread}' not found, did you add any messages to it?`)
  + }
  +
  // Capture the previous step value if there previous line included a prompt
  var previous = (step.index >= 1) ? thread[step.index - 1] : null;
  if (step.result && previous && previous.collect) {
  