BUG/MINOR: cli: add severity in "set server addr" parser

Commit c3680ec ("MINOR: add severity information to cli feedback messages")
introduced a severity level to CLI messages, but one of them was missed
on "set server addr". No backport is needed.
diff --git a/src/server.c b/src/server.c
index 3265680..11fe697 100644
--- a/src/server.c
+++ b/src/server.c
@@ -4204,6 +4204,7 @@
 	}
 	else if (strcmp(args[3], "fqdn") == 0) {
 		if (!*args[4]) {
+			appctx->ctx.cli.severity = LOG_ERR;
 			appctx->ctx.cli.msg = "set server <b>/<s> fqdn requires a FQDN.\n";
 			appctx->st0 = CLI_ST_PRINT;
 			SPIN_UNLOCK(SERVER_LOCK, &sv->lock);