stream-unix: Reduce connection failure log level from ERR to WARN.

Sometimes this just means that the daemon we're connecting to is
restarting.

Bug #13177.
Reported-by: Scott Hendricks <shendricks@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>

lib/stream-unix.c

@@ -48,7 +48,7 @@ unix_open(const char *name, char *suffix, struct stream **streamp,
 
     fd = make_unix_socket(SOCK_STREAM, true, NULL, connect_path);
     if (fd < 0) {
-        VLOG_ERR("%s: connection failed (%s)", connect_path, strerror(-fd));
+        VLOG_WARN("%s: connection failed (%s)", connect_path, strerror(-fd));
         return -fd;
     }