Add -Wall,-Werror compiler flags

src/evaluate.c

@@ -91,4 +91,7 @@ struct ucl_object *ucl_evaluate(struct ucl_scope *scope, struct ucl_object *obj)
             assert(0);
             return NULL;
     }
+
+    assert(0);
+    return ucl_error_create("Unreachable error in ucl_evaluate");
 }