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.
@takluyver ah, yeah, that's probably better actually. I'll play around with it some more (might not finish it tonight though)