Improve forum error handling

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.

lms/lib/comment_client/models.py

@@ -119,13 +119,13 @@ class Model(object):
     @classmethod
     def url(cls, action, params={}):
         if cls.base_url is None:
-            raise CommentClientError("Must provide base_url when using default url function")
+            raise CommentClientRequestError("Must provide base_url when using default url function")
         if action not in cls.DEFAULT_ACTIONS:
             raise ValueError("Invalid action {0}. The supported action must be in {1}".format(action, str(cls.DEFAULT_ACTIONS)))
         elif action in cls.DEFAULT_ACTIONS_WITH_ID:
             try:
                 return cls.url_with_id(params)
             except KeyError:
-                raise CommentClientError("Cannot perform action {0} without id".format(action))
+                raise CommentClientRequestError("Cannot perform action {0} without id".format(action))
         else:   # action must be in DEFAULT_ACTIONS_WITHOUT_ID now
             return cls.url_without_id()