Show the right error message in the console

Summary:
Just a small update to fix the error message in `Alert.js`.

In my app, I'm using https://github.com/zo0r/react-native-push-notification to show local notifications. When the notification is tapped, I'm trying to show an alert with the notification text, but this results in an error (which is not the problem I'm addressing here). Unfortunately the wrong parameter was passed to the `console.warn`, so you couldn't see what the error message was.

Before:
<img width="416" alt="bildschirmfoto 2016-07-18 um 10 50 35" src="https://cloud.githubusercontent.com/assets/666322/16910168/7b45c7ac-4cd7-11e6-8d3e-3861d2fc648c.png">
<img width="416" alt="bildschirmfoto 2016-07-18 um 10 50 43" src="https://cloud.githubusercontent.com/assets/666322/16910173/854fc0ea-4cd7-11e6-956f-93ee68a09bd6.png">

After fix:
<img width="416" alt="bildschirmfoto 2016-07-18 um 10 50 35" src="https://cloud.githubusercontent.com/assets/666322/16910168/7b45c7ac-4cd7-11e6-8d3e-3861d2fc648c.png">
<img width="416" alt="bildschirmfoto 2016
Closes https://github.com/facebook/react-native/pull/8861

Differential Revision: D3586453

fbshipit-source-id: bf81e42bd4450ef15a301e7367e94355379c442e