NoSuchKernel used to raise during KernelManager instantiation,
but it is now delayed.

Access kernel_spec to ensure it's raised where it will be caught.

Also removes a redundant warning log immediately before raising,
which prevents complete handling of NoSuchError and produces unavoidable duplicate logs.

ensures kernel is ready before running

avoids lost output if iopub isn't connected yet

and shutdown kernel when finished, rather than relying on the kernel shutting itself down