Refactoring and improvements

Comment
* Check added for empty returns
Market
* refactoring
* Checks added fro empty rpc returns
storage
* default nodes are taken from utils.get_node_list
Vote
* Check added for empty rpc calls
* Check added for Unkown key return
Exception
* UnkownKey added
Steemnoderpc
* set_next_node_on_empty_reply added, to add a check on empty returns on the next rpc call
* Refactoring
* UnkownKey exception added
Transaction
* refactoring
Unit tests
test_vote improve and check for VoteDoesNotExistsException added