Problem

If an invalid package name is given to nix-env -i or nix-env -u, an error-message is printed and nix-env exits with an exit-code !=0.

But nix-env -e silently ignores invalid package names, and always exits with exit-code 0.
But silently ignoring errors is a bad idea.

Solution

• Print an error-message for each invalid/not installed package, given to nix-env -e.
• Exit with exit code != 0, if there was any error.