Fix some abuses of the D-Bus API

dbus_g_proxy_call() needs two G_TYPE_INVALIDs in the arguments, one to
terminate the list of input variables and one to terminate the list of
output variables.  This was causing an error, which we were ignoring;
now that we don't get an error every time, print the error message and
exit instead of continuing silently in the face of errors.

Also, make sure the dbus_g_proxy_new_for_name() call succeeds, instead
of potentially passing a NULL tear_proxy to dbus_g_proxy_call().