tweak default repo path behavior

[bcumming]

v7.0.0 generates a less than helpful error message when the default uenv image repo does not exist:

> uenv image ls
error: unable to open repo: the repository /capstor/scratch/cscs/bcumming/.uenv-images does not exist

A better behavior would be:

if using default_repo
    create the repo
else if using --repo or UENV_REPO_PATH
   error message"the repository {repo} does not exist. If this is the correct location, you can create it using `uenv repo create {repo}`"