Provide sensible error handling for FVA when original problem is infeasible

[yaccos]

I have worked on an ensamble of model for which I wanted to do FVA. However, my analysis failed with the error message TypeError: unsupported operand type(s) for *: 'float' and 'NoneType' (

reframed/reframed/cobra/variability.py

Line 35 in 15a52a9

_constraints[target] = (obj_frac * solution.fobj, inf)

). I assume the error is caused by the model not being feasible such that solution.fobj is None. I would expect the FVA function to specifically check for such cases and provide a sensible error message or return a sentinent value in such cases.