Add puts() if FILESYSTEM_init() fails

Just to be helpful if someone has a failing FILESYSTEM_init(), but
doesn't know that's their issue and keeps wondering why VVVVVV just
exits with code 1.

desktop_version/src/main.cpp

@@ -143,6 +143,7 @@ int main(int argc, char *argv[])
 
     if(!FILESYSTEM_init(argv[0], baseDir, assetsPath))
     {
+        puts("Unable to initialize filesystem!");
         return 1;
     }