Meaningful error when sombody tries to invoke #execute

[t6d]

When somebody tries to invoke a protected or private, method_missing can be used to implement smarter error handling since method_missing is invoked even though the protected or private method is technically not missing but just not callable from a certain scope. Utilizing this feature, it would be possible to produce a more meaningful message explaining the user that an Operation needs to be invoked using the #perform or .perform method.