feat(forms): improved error messages

Closes #1839

modules/angular2/test/forms/directives_spec.js

@@ -0,0 +1,26 @@
+import {ddescribe, describe, it, iit, xit, expect, beforeEach, afterEach, el,
+  AsyncTestCompleter, inject} from 'angular2/test_lib';
+import {ControlGroup, ControlDirective, ControlGroupDirective} from 'angular2/forms';
+
+export function main() {
+  describe("Form Directives", () => {
+    describe("Control", () => {
+      it("should throw when the group is not found and the control is not set", () => {
+        var c = new ControlDirective(null, null);
+        expect(() => {
+          c.controlOrName = 'login';
+        }).toThrowError(new RegExp('No control group found for "login"'));
+      });
+
+      it("should throw when cannot find the control in the group", () => {
+        var emptyGroup = new ControlGroupDirective(null);
+        emptyGroup.controlOrName = new ControlGroup({});
+
+        var c = new ControlDirective(emptyGroup, null);
+        expect(() => {
+          c.controlOrName = 'login';
+        }).toThrowError(new RegExp('Cannot find control "login"'));
+      });
+    });
+  });
+}