Should spawnteract throw an error if a kernelspec is bad?

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 