Handle kernel start failures more gracefully

If a user does not have a kernel installed on their system, but they try to open a notebook with that kernel, they will get a "kernel died" error, which doesn't make a lot of sense because the kernel was never running. This pull request attempts to handle kernel start errors more gracefully, and give the user a more informative message.
Not too hard. I've updated the message to include this, it looks like this:

![image](https://cloud.githubusercontent.com/assets/83444/4429369/6abe4538-45e7-11e4-95e0-07e12e88c8fc.png)