refactor classes, add private, minor fixes

1-js/10-error-handling/2-custom-errors/1-format-error/solution.md

@@ -0,0 +1,16 @@
+```js run untrusted
+class FormatError extends SyntaxError {
+  constructor(message) {
+    super(message);
+    this.name = "FormatError";
+  }
+}
+
+let err = new FormatError("formatting error");
+
+alert( err.message ); // formatting error
+alert( err.name ); // FormatError
+alert( err.stack ); // stack
+
+alert( err instanceof SyntaxError ); // true
+```

1-js/10-error-handling/2-custom-errors/1-format-error/task.md

@@ -0,0 +1,22 @@
+importance: 5
+
+---
+
+# Inherit from SyntaxError
+
+Create a class `FormatError` that inherits from the built-in `SyntaxError` class.
+
+It should support `message`, `name` and `stack` properties.
+
+Usage example:
+
+```js
+let err = new FormatError("formatting error");
+
+alert( err.message ); // formatting error
+alert( err.name ); // FormatError
+alert( err.stack ); // stack
+
+alert( err instanceof FormatError ); // true
+alert( err instanceof SyntaxError ); // true (because inherits from SyntaxError)
+```