Add raise

Author: soutaro

lib/rbs/prototype/runtime.rb

@@ -250,6 +250,8 @@ def merge_rbs(module_name, members, instance: nil, singleton: nil)
               method = builder.build_singleton(module_name.absolute!).methods[singleton]
               method_name = singleton
               kind = :singleton
+            else
+              raise
             end
 
             if method