Improve error messages

[9999years]

Notably, this will help debug which executable failed to launch, helping to debug issues like #76

[bbigras]

👍 with this PR I had a more useful error when I was missing nix-output-monitor in my path.

❯ nix shell nixpkgs#google-chrome --impure
Error:   × Unable to launch nom shell 'nixpkgs#google-chrome' --impure --command fish
  ╰─▶ No such file or directory (os error 2)

[github-actions]

A PR to release these changes has been created, bumping the version from 1.4.5 to 1.4.6.