Make form folder optional

When the form folder didn't exist, an exception was thrown that
froze the frontend without any error message. Now it is tested
whether the form folder in a module is available and only registered
in the auotoloader if so.

refs #4407

library/Icinga/Application/Modules/Module.php

@@ -227,8 +227,9 @@ class Module
             $moduleLibraryDir = $this->getLibDir(). '/'. $moduleName;
 
             $this->app->getLoader()->registerNamespace($moduleName, $moduleLibraryDir);
-
-            $this->app->getLoader()->registerNamespace($moduleName. '\\Form', $this->getFormDir());
+            if (is_dir($this->getFormDir())) {
+                $this->app->getLoader()->registerNamespace($moduleName. '\\Form', $this->getFormDir());
+            }
         }
 
         return $this;