refactor classes, add private, minor fixes

1-js/11-async/03-promise-chaining/01-then-vs-catch/solution.md

@@ -0,0 +1,20 @@
+The short answer is: **no, they are not the equal**:
+
+The difference is that if an error happens in `f1`, then it is handled by `.catch` here:
+
+```js run
+promise
+  .then(f1)
+  .catch(f2);
+```
+
+...But not here:
+
+```js run
+promise
+  .then(f1, f2);
+```
+
+That's because an error is passed down the chain, and in the second code piece there's no chain below `f1`.
+
+In other words, `.then` passes results/errors to the next `.then/catch`. So in the first example, there's a `catch` below, and in the second one -- there isn't, so the error is unhandled.