Fix alert's error message

Summary:
`error` is not an object so `error.message` will return 'undefined'
Closes https://github.com/facebook/react-native/pull/9271

Differential Revision: D3689980

Pulled By: javache

fbshipit-source-id: 07de946f04cdff0e8013faa2c5512c51c82147b9

Examples/UIExplorer/js/GeolocationExample.js

@@ -59,7 +59,7 @@ class GeolocationExample extends React.Component {
         var initialPosition = JSON.stringify(position);
         this.setState({initialPosition});
       },
-      (error) => alert(error.message),
+      (error) => alert(error),
       {enableHighAccuracy: true, timeout: 20000, maximumAge: 1000}
     );
     this.watchID = navigator.geolocation.watchPosition((position) => {