HaikuDepot: Show alert when scheduling package action failed
HaikuDepot: Log more errors to stderr.

These errors do not necessarily need to be reported to the user via alerts.
They are more of an indication that HaikuDepot needs to be smarter when
figuring out what package actions to present at all.
HaikuDepot: Move worker thread init into method

... and call it from the second version of the MainWindow constructor.