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, 2 insertions, 1 deletions

diff --git a/lib/types.nix b/lib/types.nix
index a7f9bf1946e6..27b653ebb6c7 100644
--- a/lib/types.nix
+++ b/lib/types.nix
@@ -103,7 +103,8 @@ rec {
     path = mkOptionType {
       name = "path";
       # Hacky: there is no ‘isPath’ primop.
-      check = x: builtins.substring 0 1 (toString x) == "/";
+      # need to check isString first because otherwise toString throws an error.
+      check = x: builtins.isString x && builtins.substring 0 1 (toString x) == "/";
       merge = mergeOneOption;
     };