Throw an error if a kernel does not support kernel message spec 5

[jasongrout]

(from @blink1073) - reject the promise starting a kernel when the kernel speaks the wrong protocol.

@minrk - we can only tell what message protocol version a kernel speaks by connecting to it and asking for the kernel info. Should we have a kernelspec field saying what protocol version the kernel speaks? Or should we have a compatibility guarantee that we will always be able to start a kernel and ask for the kernel info, no matter the protocol version implemented?

[minrk]

Or should we have a compatibility guarantee that we will always be able to start a kernel and ask for the kernel info, no matter the protocol version implemented?

We already have this. The notebook server adapts to the kernel's protocol version, so frontends shouldn't ever need to care what version kernels implement.