types.nix: better error message for non-paths

This improves error messages when a set or a list is used where a path
was expected. For an example, if you used a package set (as opposed to a
single package) in systemPackages before this commit, the error was:

```
cannot coerce a list to a string, at "/home/nixpkgs/lib/types.nix":103:37
```

Now, the error message reads:

```
The option value `environment.systemPackages' in `/etc/nixos/configuration.nix' is not a list of paths.
```