The Linux installer should try to stop Jetty only if the pid file exist

Current Situation

• The Linux installer tries always to stop and start Jetty.
• Stopping makes no sense when Jetty not installed or not running
• In this case the installer logs an error of Jetty's startscript which just confuses

  Stopping Jetty: ERROR: no pid found at /path/to/jetty/base/joc.pid
  

Desired Behavior

The Linux installer tries to stop Jetty only if the pid file exist.