fix: allow workers to communicate recoverable errors

A worker may encounter an error that does not necessarily require
stopping the execution of a test script, e.g. a local worker may not
be able to load a plugin. The worker can communicate such errors
by setting a "level" property on the workerError message.

This change takes level into account for determining whether the
worker has stopped running or not.

Previous version of this code has a bug, where the stoppedError
state assigned to the worker [1] would get overridden elsewhere
meaning that the condition that determined whether all workers
exited [2] would not evaluate to true as expected. It also had
the same issue of not taking level into account, meaning that
code that handled loading plugins in local worker threads[3] should
have stopped the execution of a test early, but didn't.

1. https://github.com/artilleryio/artillery/blob/1f6e1c6d95d943f1ee6e5ef5b422f8ea70c24b85/packages/artillery/lib/launch-platform.js#L79
2. https://github.com/artilleryio/artillery/blob/1f6e1c6d95d943f1ee6e5ef5b422f8ea70c24b85/packages/artillery/lib/launch-platform.js#L242
3. https://github.com/artilleryio/artillery/blob/1f6e1c6d95d943f1ee6e5ef5b422f8ea70c24b85/packages/artillery/lib/platform/local/worker.js#L160-L167

packages/artillery/lib/launch-platform.js

@@ -60,9 +60,12 @@ class Launcher {
 
   async initWorkerEvents(workerEvents) {
     workerEvents.on('workerError', (workerId, message) => {
-      this.exitedWorkersCount++;
-
       const { id, error, level, aggregatable, logs } = message;
+
+      if (level !== 'warn') {
+        this.exitedWorkersCount++;
+      }
+
       if (aggregatable) {
         this.workerMessageBuffer.push(message);
       } else {