jupyter: if the kernel is broken, surface that fact in the graphical UI

[williamstein]

1. Break your Jupyter kernel somehow, e.g., by installing incompatible packages in ~/.local
2. In a terminal you can easily see that it is totally broken and why: jupyter console --kernel=python3, e.g.,

3. However, in a cocalc Jupyter notebook, it just gets stuck at starting for a while, then gives up.

I don't know exactly what is possible, but if nothing else, if the kernel fails to start 2-3 times, at least there could be a clear error and it could just running jupyter console --kernel=python3 or even have the output of doing so.

I'm marking this as priority high, since it's probably the most common issue overall that people hit on cocalc!