wireproto: properly implement batchable checking remoteiterbatcher (unlike remotebatcher) only supports batchable commands. This claim can be validated by comparing their implementations of submit() and noting how remoteiterbatcher assumes the invoked method has a "batchable" attribute, which is set by @peer.batchable. remoteiterbatcher has a custom __getitem__ that was trying to validate that only batchable methods are called. However, it was only validating that the called method exists, not that it is batchable. This wasn't a big deal since remoteiterbatcher.submit() would raise an AttributeError attempting to `mtd.batchable(...)`. Let's fix the check and convert it to ProgrammingError, which may not have been around when this was originally implemented. Differential Revision: https://phab.mercurial-scm.org/D317