macapp: report failure of node startup to the user

macapp: report failure of node startup to the user

in certain cases (e.g. the node.pem changed but old .furls are in private/)
the node will abort upon startup. previously it used os.abort() which in these
cases caused the mac gui app to crash on startup with no explanation.

this changes that behaviour from calling os.abort() to calling
node._abort_process(failure) which by default calls os.abort().  this allows
that method to be overridden in subclasses.

the mac app now provides and uses such a subclass of Client, so that failures
are reported to the user in a message dialog before the process exits.
this uses wx.PostEvent() with a custom event type to signal from the reactor
thread into the gui thread.