Change how fatalMessageBoxThenExit works

This avoids crashing when using Qt : we cannot call exit() safely since
QGuiApplication won’t shut down correctly. Instead throw a special
marker object and catch this in boostrap.

For an instance of this, see:

https://sourceforge.net/p/flightgear/codetickets/2070/

src/GUI/MessageBox.cxx

@@ -191,7 +191,10 @@ MessageBoxResult fatalMessageBoxWithoutExit(const std::string& caption,
     int exitStatus)
 {
     fatalMessageBoxWithoutExit(caption, msg, moreText);
-    exit(exitStatus);
+    // we can't use exit() here or QGuiApplication crashes
+    // let's instead throw a sepcial exception which we catch
+    // in boostrap.
+    throw FatalErrorException{};
 }
 
 } // of namespace flightgear

src/GUI/MessageBox.hxx

@@ -7,6 +7,12 @@
 namespace flightgear
 {
 
+// special exception class used to signal an exit. Must not inherit
+// std::exception or similar, since we want to handle it specially
+class FatalErrorException
+{
+};
+
 enum MessageBoxResult
 {
     MSG_BOX_OK,

src/Main/bootstrap.cxx

@@ -320,9 +320,10 @@ int main ( int argc, char **argv )
         flightgear::fatalMessageBoxWithoutExit("Fatal exception", e.what());
     } catch (const std::string &s) {
         flightgear::fatalMessageBoxWithoutExit("Fatal exception", s);
+    } catch (const flightgear::FatalErrorException&) {
+        // we already showed the message box, just carry on to exit
     } catch (const char *s) {
         std::cerr << "Fatal error (const char*): " << s << std::endl;
-
     } catch (...) {
         std::cerr << "Unknown exception in the main loop. Aborting..." << std::endl;
         if (errno)