CommentClientError now has sane subclasses that are meaningfully
distinct, and each subclass is handled appropriately. Errors raised by
the requests library are no longer handled by turning them into
CommentClientErrors, since there is no meaningful handling we can do,
and this way we will get more visibility into why errors are occurring.
Also, HTTP status codes from the comments service indicating client
error are correctly passed through to the client.

Also fixed up some random bugs with content_info, and changed pagination code a little. Needs comments service update for the subscribed threads API call.

Requires cs_comments_service to have the field default_sort_key on users.