mk: ensure that we have a version number before starting the download

This print an error message when the user did not choose a proper
version nu,ber when creating a new environment, instead of causing
weird errors later:

$ nv mk my_env --go-prebuilt
Creating environment: my_env ...
 * installing go-prebuilt== ...
 - please choose a version (eg. --go-prebuilt=$VERSION, see `nv ls-versions --go-prebuilt`).