Clean up object creation code

Make all of FPrintManager's struct members private,
warn of errors using a _get_error() function, as object
creation can never fail.

The only error possible shouldn't really be an error though,
as it only means that no devices will be enumerated, but it
could tell us about newly plugged devices instead.

src/main.c

@@ -237,6 +237,12 @@ int main(int argc, char **argv)
 	/* create the one instance of the Manager object to be shared between
 	 * all fprintd users */
 	manager = fprint_manager_new();
+	error = fprint_manager_get_error (manager);
+	if (error != NULL) {
+		g_error("Couldn't create manager object: %s", error->message);
+		g_error_free (error);
+		return 1;
+	}
 
 	driver_proxy = dbus_g_proxy_new_for_name(fprintd_dbus_conn,
 		DBUS_SERVICE_DBUS, DBUS_PATH_DBUS, DBUS_INTERFACE_DBUS);