replaced all 'exit's with an ExitEvent exception

The main exits are initiated from code that cannot filter this back to D_DoomMain easily so the exception is the only way to get there.

The 3 main points of exit are:

* quit/exit CCMD
* quitting the menu through ST_Endoom
* receiving a quit message on the main window.

src/console/c_cmds.cpp

@@ -101,12 +101,12 @@ bool CheckCheatmode (bool printmsg)
 
 CCMD (quit)
 {
-	if (!insave) exit(0);
+	if (!insave) throw CExitEvent(0);
 }
 
 CCMD (exit)
 {
-	if (!insave) exit(0);
+	if (!insave) throw CExitEvent(0);
 }
 
 /*

src/d_iwad.cpp

@@ -726,7 +726,7 @@ int FIWadManager::IdentifyVersion (TArray<FString> &wadfiles, const char *iwad,
 				}
 				else
 				{
-					exit(0);
+					return -1;
 				}
 				havepicked = true;
 			}
@@ -793,6 +793,7 @@ int FIWadManager::IdentifyVersion (TArray<FString> &wadfiles, const char *iwad,
 const FIWADInfo *FIWadManager::FindIWAD(TArray<FString> &wadfiles, const char *iwad, const char *basewad, const char *optionalwad)
 {
 	int iwadType = IdentifyVersion(wadfiles, iwad, basewad, optionalwad);
+	if (iwadType == -1) return nullptr;
 	//gameiwad = iwadType;
 	const FIWADInfo *iwad_info = &mIWadInfos[iwadType];
 	if (DoomStartupInfo.Name.IsEmpty()) DoomStartupInfo.Name = iwad_info->Name;

src/d_main.cpp

@@ -2459,6 +2459,7 @@ static int D_DoomMain_Internal (void)
 			iwad_man = new FIWadManager(basewad, optionalwad);
 		}
 		const FIWADInfo *iwad_info = iwad_man->FindIWAD(allwads, iwad, basewad, optionalwad);
+		if (!iwad_info) return 0;	// user exited the selection popup via cancel button.
 		gameinfo.gametype = iwad_info->gametype;
 		gameinfo.flags = iwad_info->flags;
 		gameinfo.ConfigName = iwad_info->Configname;
@@ -2847,7 +2848,11 @@ int D_DoomMain()
 	{
 		ret = D_DoomMain_Internal();
 	}
-	catch (std::exception &error)
+	catch (const CExitEvent &exit)	// This is a regular exit initiated from deeply nested code.
+	{
+		ret = exit.Reason();
+	}
+	catch (const std::exception &error)
 	{
 		I_ShowFatalError(error.what());
 		ret = -1;

src/posix/cocoa/st_start.mm

@@ -39,6 +39,7 @@
 #include "doomtype.h"
 #include "st_console.h"
 #include "st_start.h"
+#include "doomerrors.h"
 
 
 FStartupScreen *StartScreen;
@@ -174,5 +175,5 @@ void ST_Endoom()
 	extern void I_ShutdownJoysticks();
 	I_ShutdownJoysticks();
 
-	exit(0);
+	throw CExitEvent(0);
 }

src/posix/sdl/i_input.cpp

@@ -47,6 +47,7 @@
 #include "g_game.h"
 #include "g_levellocals.h"
 #include "utf8.h"
+#include "doomerrors.h"
 
 
 static void I_CheckGUICapture ();
@@ -297,7 +298,7 @@ void MessagePump (const SDL_Event &sev)
 	switch (sev.type)
 	{
 	case SDL_QUIT:
-		exit(0);
+		throw CExitEvent(0);
 
 	case SDL_WINDOWEVENT:
 		extern void ProcessSDLWindowEvent(const SDL_WindowEvent &);

src/posix/sdl/st_start.cpp

@@ -350,5 +350,5 @@ bool FTTYStartupScreen::NetLoop(bool (*timer_callback)(void *), void *userdata)
 void ST_Endoom()
 {
 	I_ShutdownJoysticks();
-	exit(0);
+	throw CExitEvent(0);
 }

src/win32/i_input.cpp

@@ -739,7 +739,7 @@ void I_GetEvent ()
 	while (PeekMessage (&mess, NULL, 0, 0, PM_REMOVE))
 	{
 		if (mess.message == WM_QUIT)
-			exit(mess.wParam);
+			throw CExitEvent(mess.wParam);
 
 		if (GUICapture)
 		{

src/win32/st_start.cpp

@@ -611,7 +611,7 @@ int RunEndoom()
 void ST_Endoom()
 {
 	int code = RunEndoom();
-	exit(code);
+	throw CExitEvent(code);
 
 }