Referring to a nonexistent overlay in an input results in an error when attempting to display a consumable error message

[mcdonc]

On the python-rewrite branch, referring to a nonexistent overlay within an input tries to show a consumable error, telling the user which overlays actually exist, but the code in .devenv.flake.nix errors out when it tries to do so.

Putting this here so I can remember about it, I'll put more information in here when I get my system into a place where I can produce the error again.