This reverts commit 9e6a3ebc6b ("Make
fatalMessageBox() end with std::abort() and declare it [[noreturn]]").
After reflexion, it seems better to let fatalMessageBox() return,
because there is existing code that appears to be relying on this aspect
to do some work after having called fatalMessageBox() (cf. main() in
bootstrap.cxx). Also, the way of exiting from fatalMessageBox() after
commit 9e6a3ebc6b (std::abort()) was probably too brutal for a
controlled exit---as opposed to a terminate handler.
This allows us to display a platform-native dialog for problems
which occur early in startup (before we can show a PUI/Canvas dialog).
In particular this improves feedback where FG_HOME, FG_DATA or
aircraft selection is wrong, all of which happen very early in startup.
