Prompt the user to restart the kernel when running a cell against a dead kernel

[DonJayamanne]

Run some code, and assume the kernl dies.
Next the user attempts to run a cell again
This is what you get (when kernel is dead)

Looks horrible, I think we should detect this and just inform the user the kernel is dead and ask them to restart the kernle (yes/no).

Else we just leave the user to figure out whats wrong & what they need to do.

[DonJayamanne]

I think we should restart automatically, that's what happens in Jupyter.
Also if we're using jupyter (instead of Raw kernels) that's what will happen.

Hence revised suggestion is to display a message indicating the kernel is being restarted automatically.

[DonJayamanne]

Done
Testing:

• Create a notebook
• Run a cell with the code

import os
os.getpid()

• Kill the kernel prcoess (pid printed above)
• Try to run the same or anotehr cell in the same notebook, user should be prompted to restart the kernel (notify them the kernel is dead)

[rchiodo]

/verified