raft: remove panic when we see a proposal with no leader.

This panic can never be reached when using raft.Node, because we only
read from propc when there is a leader. However, it is possible to see
this error when using raft the raft object directly (as in MultiNode),
and in this case it is better to simply drop the proposal (as if we had
sent it to a leader that immediately vanished).

Add an error return to MemoryStorage.Append for consistency.
a follower should always have a leader after it receives the first message from a leader.
but i do not think there is an easy way to ensure that right now.