* guix-jupyter-kernel.scm (reply-execute-request): Wrap
'create-environment' calls in 'guard' and send a proper HTML reply upon
'package-not-found-error?' and 'output-not-found-error?'.
* guix/jupyter/environment.scm (&environment-error)
(&package-not-found-error, &output-not-found-error): Export accessors.