exchange: refactor bundle specification parsing The old code was tailored to `hg bundle` usage and not appropriate for use as a general API, which clone bundles will require. The code has been rewritten to make it more generally suitable. We introduce dedicated error types to represent invalid and unsupported bundle specifications. The reason we need dedicated error types (rather than error.Abort) is because clone bundles will want to catch these exception as part of filtering entries. We don't want to swallow error.Abort on principle.