Error message only when initialization hadn't been done

In a multithread environment, sogo_backend_init is registering the thread
only for the first one that calls this function, then the others threads
(even if they call sogo_backend_init) won't be registered because
moduleInitialized was YES.

We just want to ensure sogo_backend_init is called at least once (per process,
not per thread).
increase log level of debug message
Merge pull request #22 from Zentyal/jgarcia/register-thread

Jgarcia/register thread