Updated indexing response error handler

[notbakaneko]

Fix some issues found during testing with the response and exception not being handled by the correct case; also sends some error metrics to datadog.

Uses IsValid on response to determine if the request was successful or not. With exceptions, it was messy to figure out which cases should be handled or not. (Other requests like creating indices still use exceptions; in those cases, every error should be considered fatal)

In the event of an error, ServerError may or may not be set depending on what and which stage in the server's processing pipeline an error was triggered. ItemsWithErrors will generally be empty if the processing doesn't get to the index level.

Closed indices will result in either index_closed_exception or cluster_block_exception in the return response items.

Waiting for a server to come back online is removed and waiting for the option to re-queue items without failing them and depleting all the retries. ppy/osu-queue-processor#16

requires

• Move organizing buffer items into ProcessableItemsBuffer construction #120