chain: ensure eventType from ZMQ response is human-readable
chain: ensure eventType from ZMQ response is human-readable

In this commit, we fix a small issue where it's possible that we read a
malformed message from the ZMQ connection to bitcoind due to it shutting
down. To fix this, we ensure that the event type is human readable
before attempting to log it.

btcsuite/btcwallet,btcsuite/btcwallet