Catch missing kernel

[light-matters]

If Wolframite is not connected to a kernel then we should catch this when performing any operation, return an informative error message and advise the user to use the 'start' command.

Currently, we just get

1. Unhandled java.lang.AssertionError
   Assert failed: (proto/kernel-link? jlink-instance)
...

This is actually fairly descriptive, but advice on how to start the kernel properly would still be better.