Add Serbian error message translation

Translated by: Nikola Smolenski <smolensk eunet.yu>
Reviewed by: Danilo Šegan <danilo gnome.org>


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.0.x@219385 13f79535-47bb-0310-9956-ffa450edef68