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.
```

1 files changed, 1 insertions, 1 deletions

diff --git a/nixos/modules/config/system-path.nix b/nixos/modules/config/system-path.nix
index 62390f452ba..d7979593f40 100644
--- a/nixos/modules/config/system-path.nix
+++ b/nixos/modules/config/system-path.nix
@@ -57,7 +57,7 @@ in
     environment = {
 
       systemPackages = mkOption {
-        type = types.listOf types.path;
+        type = types.listOf types.package;
         default = [];
         example = literalExample "[ pkgs.firefox pkgs.thunderbird ]";
         description = ''