XWiki starts up incorrectly if temporary directory already exists at time of startup

if the xwiki-temp temporary directory already exists at time of XWiki startup, XWiki will fail to create the directory and log an error.

Worse, it will continue operating with the incorrect xwiki-temp directory and any files which happen to be inside of it will not be differentiated from files which were created while XWiki ran.

[http://localhost:8080/xwiki/bin/download/XWiki/AdminSheet/configuration.png] ERROR o.x.e.i.ServletEnvironment     - Configured temporary directory [/tmp/jetty-0.0.0.0-8080-xwiki-_xwiki-any-7578883752776679013.dir/xwiki-temp] could not be created, check permissions.