Raise exception when objective value is missing in objective function

[jpzk]

I think it's useful to raise a exception when the user accidently forgets to return the objective value in the objective function 'f'. It just happened to me.

[renke]

Mh, I am not sure if we should raise an exception to the user or just call on_error in the caller. I will think about a suitable solution to this problem. The former is probably better unless we expect objective function to return None for some arguments.

[jpzk]

This sounds like a good solution.