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.

src/zmq/zmqutil.cpp

@@ -12,5 +12,5 @@
 
 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));
 }