engine: be more verbose on why the engine is going to be shut down

engine/platform/posix/crash_posix.c

@@ -197,8 +197,7 @@ static void Sys_Crash( int signal, siginfo_t *si, void *context)
 		CL_Crashed();
 	host.status = HOST_CRASHED;
 
-
-	Sys_Quit();
+	Sys_Quit( "crashed" );
 }
 
 void Sys_SetupCrashHandler( void )

engine/platform/posix/sys_posix.c

@@ -148,7 +148,9 @@ void Posix_Daemonize( void )
 
 static void Posix_SigtermCallback( int signal )
 {
-	Sys_Quit();
+	string reason;
+	Con_Printf( reason, sizeof( reason ), "caught signal %d", signal );
+	Sys_Quit( reason );
 }
 
 void Posix_SetupSigtermHandling( void )

engine/platform/sdl/events.c

@@ -498,7 +498,7 @@ static void SDLash_EventHandler( SDL_Event *event )
 		break;
 
 	case SDL_QUIT:
-		Sys_Quit();
+		Sys_Quit( "caught SDL_QUIT" );
 		break;
 #if SDL_VERSION_ATLEAST( 2, 0, 0 )
 	case SDL_MOUSEWHEEL:

engine/platform/win32/crash_win.c

@@ -305,7 +305,7 @@ static long _stdcall Sys_Crash( PEXCEPTION_POINTERS pInfo )
 			if( host_developer.value <= 0 )
 			{
 				// no reason to call debugger in release build - just exit
-				Sys_Quit();
+				Sys_Quit( "crashed" );
 				return EXCEPTION_CONTINUE_EXECUTION;
 			}