Update to set a provider init global error handler [ref #6]

I've added a global ``providerInitError`` to handle errors during the dbus session init in *keyring_linux* provider. That way, keyring ``Get`` and ``Set`` methods are able to check on errors during the init (without logging).

I think something cleaner could have been made in terms of design. But I tried to stick to the way you made it to fit with the whole codebase.

Let me know what you think.