In addition to simplifying the code, this ensures that
G_MESSAGES_DEBUG is honored, which in turn shuts up the
logging of debug messages by default, and avoids filling
the journal of everyone who's running a recent dbus-daemon.
The following map was used when converting log levels:
 - debug -> debug
 - notice -> message
 - info -> info
 - warning -> warning
 - error -> critical (because g_error() aborts the application)

https://bugzilla.gnome.org/show_bug.cgi?id=723926