Raise error when we can't found VboxManage at GNS3 VM startup

Fix #824

gns3server/compute/virtualbox/__init__.py

@@ -93,6 +93,9 @@ class VirtualBox(BaseManager):
             vboxmanage_path = self.vboxmanage_path
             if not vboxmanage_path:
                 vboxmanage_path = self.find_vboxmanage()
+            if not vboxmanage_path:
+                raise VirtualBoxError("Could not found VBoxManage")
+
             command = [vboxmanage_path, "--nologo", subcommand]
             command.extend(args)
             command_string = " ".join(command)