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.
This is a much needed fix! I wonder about the choice of words: "We..." ... who is that "we"?  Sounds like some little gnomes in there :) Perhaps making kernel the (non)actor: "The kernel was unable to start. ...."