Handle NoSuchKernel errors more gracefully
Show the user a different notification
Report the exact error that occurred
Remove 'we' from message
Better user experience when kernel isn't found
Use 501 error code instead of 400
Return a proper JSON object
Always show the modal dialog, and have a fallback generic message
Don't pop up modal dialog
More descriptive short message
Use codemirror for error messages
Merge pull request #6558 from jhamrick/kernel-start-failure

Handle kernel start failures more gracefully