Return non-zero on display usage

`Icingacli` should return non-zeno status code when display usage messages
 for example for invalid commands

https://github.com/Icinga/icinga2/issues/6585#issuecomment-418681789

It is good for automation and helps mitigate typing errors

library/Icinga/Application/Cli.php

@@ -146,11 +146,12 @@ class Cli extends ApplicationBootstrap
     {
         $loader = $this->cliLoader();
         $loader->parseParams();
-        $loader->dispatch();
+        $result = $loader->dispatch();
         Benchmark::measure('All done');
         if ($this->showBenchmark) {
             Benchmark::dump();
         }
+        if ($result === FALSE) exit(1);
     }
 
     protected function dispatchEndless()