lib/modules: Improve errors involving pushDownProperties

If an attrset option was given a definition like:

    { system = lib.mkIf true false; }

Before this change, we get the non-explanatory error message:

    error: expected a set but found a Boolean: true

With the stack trace having nothing to do with the module involved.
After this change, we get the better:

    error: In module `[...]', you're trying to define a value of type `bool' [...]

Which names the actual module involved.

lib/modules.nix

@@ -1320,13 +1320,24 @@ let
     : 1\. Function argument
   */
   pushDownProperties =
+    let
+      mapAttrsIfAttrs =
+        f: val:
+        if isAttrs val then
+          mapAttrs f val
+        else
+          # This does not actually work, since arriving here means we have e.g.
+          # (lib.mkIf cond nonAttrs), while an attrset is expected. However,
+          # avoiding the mapAttrs call here gives better errors later.
+          val;
+    in
     cfg:
     if cfg._type or "" == "merge" then
       concatMap pushDownProperties cfg.contents
     else if cfg._type or "" == "if" then
-      map (mapAttrs (n: v: mkIf cfg.condition v)) (pushDownProperties cfg.content)
+      map (mapAttrsIfAttrs (n: v: mkIf cfg.condition v)) (pushDownProperties cfg.content)
     else if cfg._type or "" == "override" then
-      map (mapAttrs (n: v: mkOverride cfg.priority v)) (pushDownProperties cfg.content)
+      map (mapAttrsIfAttrs (n: v: mkOverride cfg.priority v)) (pushDownProperties cfg.content)
     # FIXME: handle mkOrder?
     else
       [ cfg ];