Correct validators so that existing error messages would look correct (#3668)

app/validators/unreserved_username_validator.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+class UnreservedUsernameValidator < ActiveModel::Validator
+  def validate(account)
+    return if account.username.nil?
+    account.errors.add(:username, I18n.t('accounts.reserved_username')) if reserved_username?(account.username)
+  end
+
+  private
+
+  def reserved_username?(value)
+    return false unless Setting.reserved_usernames
+    Setting.reserved_usernames.include?(value.downcase)
+  end
+end