add warning when reward type is 'reward_v2'

[olb1ue]

Recently blockchain_node has been struggling to keep up with the chain when store_json is on (see issue helium/blockchain-node#121). This causes rewards transactions to return with only account-level data versus gateway-level, which is not ideal for the etl-lite. When this is happening, rewards come in with type reward_v2. We need to see a warning in the logs any time rewards are being loaded with type reward_v2, since this means we're not getting all the details we are expecting from blockchain_node.