tasks: make error a json

also, handle case where we never got to handle task exit cleanly

src/taskworker.js

@@ -57,7 +57,12 @@ initialize(function (error) {
 
         const progressCallback = (progress, cb) => tasks.update(taskId, progress, cb || NOOP_CALLBACK);
         const resultCallback = (error, result) => {
-            const progress = { percent: 100, result: result || null, errorMessage: error ? error.message : null };
+            // Error object has properties with enumerable: false (https://mattcbaker.com/posts/stringify-javascript-error/)
+            const progress = {
+                percent: 100,
+                result: result || null,
+                error: error ? JSON.parse(JSON.stringify(error, Object.getOwnPropertyNames(error))) : null
+            };
 
             tasks.update(taskId, progress, () => process.exit(error ? 50 : 0));
         };