Clear error message when rendering a nonvalid component

Author: matnel

src/core/ReactCompositeComponent.js

@@ -860,7 +860,8 @@ var ReactCompositeComponentMixin = {
     }
     invariant(
       ReactComponent.isValidComponent(renderedComponent),
-      '%s.render(): A valid ReactComponent must be returned.',
+      '%s.render(): A valid ReactComponent must be returned. You may have ' +
+      'returned null, undefined, an array or some other invalid object.',
       this.constructor.displayName || 'ReactCompositeComponent'
     );
     return renderedComponent;