Seems to me that the current error messages and both accurate and reasonable.
if you create /home/adavidoff/git/marsh/deps/pkgs/, i'll use it for local dependencies
/home/adavidoff/git/marsh/deps/packages_official.json not found
couldn't get nimble's package list from /home/adavidoff/git/marsh/deps/pkgs/
Exception: /home/adavidoff/git/marsh/deps/ is missing; can't run nimble
crashing because something bad happened
Seems to me that the current error messages and both accurate and reasonable.