For Retryable and DataLoss errors, EventListener::OnBackgroundError status is not set on the way in or checked on the way out

[mrambacher]

This was found by code inspection.

According to the documentation, the "bg_error" status is called with a non-OK status and the callback can set this to an "OK" value to suppress the error. By code inspection, it appears that it is always called with an OK status (s is not set to the new error) and that status is never checked or used as part of the return status (s is not checked for).

Expected behavior

Actual behavior

Steps to reproduce the behavior

The error_handler_fs_test shows calls to this handler passing in OK even when the underlyin Status is not OK.