engine: accept out-of-order error reply messages
Due to a quirk of the implementation of the D-Bus daemon (see upstream
bug https://bugs.freedesktop.org/show_bug.cgi?id=59780 for details)
error messages can arrive out of sequence with repsect to the order of
the method calls that they are in reply to.
This may or may not be a violation of the strict-ordering principle of
D-Bus (which does not actually appear to be written into the spec) but
either way, we should dial down our assumptions about ordering since the
D-Bus implementation that everyone is using does not currently work this
way.
https://bugzilla.gnome.org/show_bug.cgi?id=687120