Fixes for compositor replacement

* When unmanaging a screen, stop redirecting subwindows explicitly,
  so that we do that before destroying the window manager selection
  window.
* Improve comment in the retry code
* When exiting because the previous compositor couldn't be replaced,
  don't g_error() and drop a core file.

https://bugzilla.gnome.org/show_bug.cgi?id=653121