MNT/BUG: single _AiocaShim.get('pv') should return a CommunicationError in the same way as list version

[tangkong]

Current Behavior

Currently the single-pv version raises the communication error, while the list version returns a list of such errors.

Expected Behavior

The two should act similarly

Context / environment

poking around #73 , this is something I saw in the past but never ironed out

Steps to Reproduce (for bugs)

1. from superscore.control_layers import ControlLayer; cl = ControlLayer()
2. res_single = cl.get('nonsense:non:working:pv') (should raise)
3. res_list = cl.get(['nonsense', 'nonsense2']) (should return list)
4. Compare res_single, res_list

Suggested Solution

Catch the exception in the single version and return it?