Factor error and success methods from services.

Created by: cirosantilli

The most important factorization here is to fix a single status: :error and status: :sucess keys for all services.

The DeleteBranch service had already started diverging as is used the key :state instead of :status.

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.

The next step would be to have an uniform error reporting across all services: some create services currently return the created object with errors added by .errors.add. Maybe we should just follow that for all services as it is cleaner than playing with hashes.