refactor classes, add private, minor fixes

1-js/11-async/04-promise-error-handling/01-error-async/solution.md

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+The answer is: **no, it won't**:
+
+```js run
+new Promise(function(resolve, reject) {
+  setTimeout(() => {
+    throw new Error("Whoops!");
+  }, 1000);
+}).catch(alert);
+```
+
+As said in the chapter, there's an "implicit `try..catch`" around the function code. So all synchronous errors are handled.
+
+But here the error is generated not while the executor is running, but later. So the promise can't handle it. 

1-js/11-async/04-promise-error-handling/01-error-async/task.md

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+# Error in setTimeout
+
+What do you think? Will the `.catch` trigger? Explain your answer.
+
+```js
+new Promise(function(resolve, reject) {
+  setTimeout(() => {
+    throw new Error("Whoops!");
+  }, 1000);
+}).catch(alert);
+```