Give better error message when no workers have been registered.

[robertnishihara]

This also reduces the timeout from one minute to 5 seconds when you submit a function that hasn't been registered.

Now, if you do

import ray
ray.init(start_ray_local=True, num_workers=0)

@ray.remote
def f():
  return 1

f.remote()

It will throw the exception

---------------------------------------------------------------------------
error                                     Traceback (most recent call last)
<ipython-input-4-520f46ece26b> in <module>()
----> 1 f.remote(3)

/Users/rkn/Workspace/ray/lib/python/ray/worker.pyc in func_call(*args, **kwargs)
   1111           # the usual behavior of immutable remote objects.
   1112           return func(*copy.deepcopy(args))
-> 1113         objectids = _submit_task(func_name, args)
   1114         if len(objectids) == 1:
   1115           return objectids[0]

/Users/rkn/Workspace/ray/lib/python/ray/worker.pyc in _submit_task(func_name, args, worker)
   1056   which cannot be serialized.
   1057   """
-> 1058   return worker.submit_task(func_name, args)
   1059 
   1060 def _mode(worker=global_worker):

/Users/rkn/Workspace/ray/lib/python/ray/worker.pyc in submit_task(self, func_name, args)
    416     args = [arg if isinstance(arg, raylib.ObjectID) else put(arg, worker=self) for arg in args]
    417     task_capsule = raylib.serialize_task(self.handle, func_name, args)
--> 418     objectids = raylib.submit_task(self.handle, task_capsule)
    419     return objectids
    420 

error: No workers have registered with the scheduler, so this function cannot be run.