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
s/long thing/set_state
s/_handle_receive_state/set_state in tests too
s/apply_update/set_state
remove nbformat.v4 spec files

nbformat 4 won't be in IPython 3.0
msgspec 5: stream.data -> stream.text
Use codemirror for error messages
Clarify escaping for $.

Repeat that $$ is used when a literal $ is desired.
Merge pull request #6558 from jhamrick/kernel-start-failure

Handle kernel start failures more gracefully
Merge pull request #6582 from jdfreder/symmetric_state

Symmetric widget get/set_state
Surround $ with backticks