message: zmq: use msg: prefix over errno= in zmqError
  zmq_strerror() converts the passed errno into a description, meaning
  currently you have output like: "errno=No such file or directory".
  Using msg: would seem to make more sense here.
diff: void zmqError(const std::string& str)
  {
  - LogPrint(BCLog::ZMQ, "zmq: Error: %s, errno=%s\n", str, zmq_strerror(errno));
  + LogPrint(BCLog::ZMQ, "zmq: Error: %s, msg: %s\n", str, zmq_strerror(errno));
  }
  