Change warn -> error in people.report_late_add().

This commit is easy to revert if we want to tone down errors
to warnings for the short term, while our codepath still does
proper handling for adding users when they come in messages.

static/js/people.js

@@ -733,7 +733,7 @@ exports.report_late_add = function (user_id, email) {
     // types of realms.
     var msg = 'Added user late: user_id=' + user_id + ' email=' + email;
 
-    blueslip.warn(msg);
+    blueslip.error(msg);
 };
 
 exports.extract_people_from_message = function (message) {