Resolve inconsistencies in how Egret handles infeasiblity or other solver issues

[bknueven]

egret.common.solver_interface._solve_model raises an exception if a solver terminates with a 'not good' termination condition. Conversely, the lazy PTDF solve loops do not raise exceptions (though likely cause them) if a solver terminates with an unsafe condition.

What should the default behavior of Egret be? Should we throw exceptions if a model is infeasible, or attempt to do something more graceful?