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.
Ah, no, I haven't tried that, but that would be much better. I'll see how it looks.