Reduce size of internal MonoErrors by 3 pointers, while trying to be compatible with...

Reduce size of internal MonoErrors by 3 pointers, while trying to be compatible with old size.(mono/mono#16218)

There is an incompatibility here.

If Mono hands off an InternalError and it is accessed
like an ExternalError, like memset(sizeof()), bad.

Hopefully nobody does that.

There is also compatability -- embedders can continue to embed MonoError
in their own data. Its size and hopefully alignment is unchanged.

Commit migrated from https://github.com/mono/mono/commit/cf0ce4f55ad92529bf969bef1cbf5cc6a502fdef