Fix compilation error when cgroups v2 is not being compiled.

This closes #575
diff --git a/src/slave/containerizer/mesos/linux_launcher.cpp b/src/slave/containerizer/mesos/linux_launcher.cpp
index cbc883b..7682a56 100644
--- a/src/slave/containerizer/mesos/linux_launcher.cpp
+++ b/src/slave/containerizer/mesos/linux_launcher.cpp
@@ -786,7 +786,7 @@
 
   return cgroups2::destroy(cgroup);
 #else
-  return Error("cgroups2 is not enabled");
+  return Failure("cgroups2 is not enabled");
 #endif // ENABLE_CGROUPS_V2
 }