[Tools] - Options --set-var error when a variable does not exists can be caught more properly

fusiondirectory-plugins-manager --set-var abc=.
PHP Fatal error:  Uncaught Exception: Var "abc" does not exists. Use --list-vars to get the list of vars. in /usr/share/php/FusionDirectory/FusionDirectory/Cli/VarHandling.php:107
Stack trace:
#0 [internal function]: FusionDirectory\Tools\PluginsManager->cmdSetVar()
#1 /usr/share/php/FusionDirectory/FusionDirectory/Cli/Application.php(198): call_user_func()
#2 /usr/share/php/FusionDirectory/FusionDirectory/Tools/PluginsManager.php(119): FusionDirectory\Cli\Application->runCommands()
#3 /usr/sbin/fusiondirectory-plugins-manager(29): FusionDirectory\Tools\PluginsManager->run()
#4 {main}
  thrown in /usr/share/php/FusionDirectory/FusionDirectory/Cli/VarHandling.php on line 107

Error message is set already but can be handled in a more user-friendly way.