Fix double error dialog if data.zip is missing

A minor gripe, but one thing I didn't notice with commit
b4579d88d3 is that it now results in two
dialogs if data.zip is missing: The first being the "data.zip is
missing" dialog, and the second is the generic "Unable to initialize
filesystem" dialog.

So just bail early if data.zip can't be found, it's going to take the
error path in main() and also bail regardless.

desktop_version/src/FileSystemUtils.cpp

@@ -342,6 +342,7 @@ int FILESYSTEM_init(char *argvZero, char* baseDir, char *assetsPath, char* langD
             "\nor get it from the free Make and Play Edition.",
             NULL
         );
+        VVV_exit(1);
         return 0;
     }