Show warning when a path passed by NIX_PATH or -I does not exist.

[rbvermaa]

Nix will silently ignore entries of NIX_PATH or -I when they do not exist. It would make sense to produce a warning for this, as this usually is a user error.

[edolstra]

A slight problem with this is that profile.sh on NixOS sets $NIX_PATH to several entries that might not exist (such as /etc/nixos/nixpkgs).

[rbvermaa]

We could just add those paths only if they exist right?