rpcserver: Avoid error in handleRebroadcastWinners.
rpcserver: Avoid error in handleRebroadcastWinners.

This downgrades an error when rebroadcasting the winning tickets to a
logged warning.

This is necessary because it is a legitimate state to have a chain tip
header returned by the Chain.TipGeneration() while the actual block data
is not yet available for the LotteryDataForBlock to complete.

Prior to this change, a voting wallet connecting to a dcrd instance with
multiple chain tip headers without the associated data would be unable
to re-start voting for blocks due to the associated RPC call failing.
decred/dcrd,decred/dcrd