macapp: simplify node startup failure reporting

macapp: simplify node startup failure reporting

1. changed the node's exit-on-error behaviour. rather than logging debug and
then delegating to self for _abort_process() instead simply delegate to self
_service_startup_failed(failure) to report failures in the startup deferred
chain. subclasses then have complete control of handling and reporting any
failures in node startup.

2. replace the convoluted wx.PostEvent() glue for posting an event into the
gui thread with the simpler expedient of wx.CallAfter() which is much like
foolscap's eventually() but also thread safe for inducing a call back on the
gui thread.