Improve error message on built in policy parsing failure

Author: linus-mullvad

src/policy/builtin.rs

@@ -40,7 +40,7 @@ macro_rules! builtin_policy {
                     $name,
                     ".builtin-policy"
                 )))
-                .unwrap()
+                .expect(concat!("Failed to parse built-in policy: ", $name))
             }),
         }
     };