fix(utils): provide clear error for missing role files

lib/utils.nix

@@ -1,6 +1,24 @@
 { ... }:
 
+let
+  mkSysRole = role: 
+    let
+      path = ../roles + "/${role}.nix";
+    in
+    if builtins.pathExists path then
+      import path
+    else
+      throw "System role '${role}' not found at ${toString path}";
+  
+  mkHomeRole = role:
+    let
+      path = ../roles/home + "/${role}.nix";
+    in
+    if builtins.pathExists path then
+      import path
+    else
+      throw "Home role '${role}' not found at ${toString path}";
+in
 {
-  mkSysRole = role: import (../roles/${role}.nix);
+  inherit mkSysRole mkHomeRole;
-  mkHomeRole = role: import (../roles/home/${role}.nix);
 }