Scheduler: First-fit algorithm throws exception for empty device list

[marusshi]

Scheduler.js:282 : devices[0] is undefined

tasks.forEach(function(task){
	var most_space = Object.keys(mapping).reduce(function(acc, id){
		return (mapping[id].available_memory > mapping[acc].available_memory) ? id : acc;
	}, devices[0].id); 

Error thrown:

     TypeError: Cannot read property 'id' of undefined

[jungkumseok]

If the device list is empty, that part of the code should not even be executed. I think we should throw a "no solution" error if the given device list is empty