Status message for converged models

[brandmaier]

When printing a fitted model as in print(model_fitted), a converged model prints the line:

 * Status: success

Shouldn't this rather be

 * Status: converged

[Maximilian-Stefan-Ernst]

That would be nicer, but the way it works is that we print the message the optimization backend returns. Some backends have quite a variety of status messages, and we want to make it as easy as possible for users to also use backends we have not thought of before - so I think passing trough the backend's flag is the most sensible option.