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.
I guess it's to scroll right ? 

Have you tried a : 
```
var cm = CodeMirror(msg, {
  value: traceback,
  mode:  "python",
  readOnly : true
});
```
Should looks nicer. But can be done later.