fix(env): Throw if nested describe has no children.

Users would like an error if it() is acciddently moved within a before/afterEach/All function.
The it() function calls ensureIsNotNested to report such an error. But if the user has no
other it() functions in the Suite, it() and thus ensureIsNotNested() is never called.

Here we check nested Suites for children; if none are found we throw.

spec/core/EnvSpec.js

@@ -133,6 +133,12 @@ describe('Env', function() {
         'describe expects a function argument; received [object Function]'
       );
     });
+
+    it('throws an error when it has no children', function() {
+      expect(function() {
+        env.describe('done method', function() {});
+      }).toThrowError('describe with no children (describe() or it())');
+    });
   });
 
   describe('#it', function() {