Should spawnteract throw an error if a kernelspec is bad?

[BenRussert]

hydrogen uses launchSpec to start kernels. Currently, if a user's kernelspec is bad, ( e.g. argv[0] doesnt exist ) there is no error thrown by default.

Should we handle this in hydrogen/clients or here in spawnteract?

xref: https://github.com/nteract/hydrogen/issues/1538 cc @kylebarron