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.
Previously, AJAX calls would return 400, and page views and attempts
to load inline discussions would return 404 if communication with the
comments service failed. Now such problems cause a 500 status code.
