Notify error msg when empty project name inserted

Fixes https://github.com/dart-lang/spark/issues/1325
@ussuri 
CC @dinhviethoa

TEST=Click `Create` button without entering project name.

![image](https://f.cloud.github.com/assets/756988/2413478/c90ff022-aade-11e3-9a37-75db414a4a76.png)