CASCache methods can throw RpcError but don't declare it

Summary

As @phildawson pointed out, some methods such as push_directory declare that they throw CASErrors, but they can also throw grpc.RpcError raised by methods on remote_execution_pb2.

    # Raises:                                                                                                                                                                                               
    #     (CASError): if there was an error           

(Ignoring the standard reproduction steps as this is a documentation issue)

Possible fixes

Either change the documentation comments or handle RpcError inside CASCache.