Two changes to how simulated clusters handle having zero stars were made.

• When zero stars are simulated, ocelot.simulate.SimulatedCluster now raises a NotEnoughStarsError (defined in ocelot.simulate.errors), making it easier to catch when no stars were simulated.
• When cluster pre-pruning causes zero stars to be simulated, ocelot.simulate.SimulatedCluster now also raises a NotEnoughStarsError (previously, this behaviour was undefined.)