Generate error/warning if project variable is used but not defined

[florianesser]

If a project variable is used, e.g. in an Assign cell, but the variable is not defined, no warning or error is generated.

[github-actions[bot]]

This issue has been automatically marked as stale because it has not had activity in the last 60 days. It will be closed in two weeks if no further activity occurs. Thank you for your contributions.