LibJS: Implement Error.prototype.name setter (#1776)

The MDN example for creating a custom error type in javascript uses:

    function CustomError(foo, message, fileName, lineNumber) {
        var instance = new Error(message, fileName, lineNumber);
        instance.name = 'CustomError';
        instance.foo = foo;
        Object.setPrototypeOf(instance, Object.getPrototypeOf(this));
        return instance;
    }

The name property on the Error prototype needs to be settable for
this to work properly.

Libraries/LibJS/Tests/Error.prototype.name.js

@@ -0,0 +1,12 @@
+try {
+    var changedInstance = new Error("");
+    changedInstance.name = 'NewCustomError';
+    assert(changedInstance.name === "NewCustomError");
+
+    var normalInstance = new Error("");
+    assert(normalInstance.name === "Error");
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e.message);
+}