Factor error and success methods from services.

The most important factorization here is to fix a single `status: :error` and `status: :sucess` keys for all services.

For the error, it was also possible to factor out the `message` as it is used on all returns. The same was not possible for `success`.

The initialize signature was modified in order to allow to use inheritance. This also makes the execute function parameters lists shorter since `project` and `current_user` now go on the initializer like all other services.