Catch BulkIndexError exception in async_bulk. resolves #864

[bshakur8]

fix #864

async_bulk can raise BulkIndexError despite setting raise_on_error=False, something that prevents the rest of the bulk from running. Besides, as mentioned in #864 it prevents using additional flags like max_retries.

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

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