Determine migration path for instrument_errors parameter to MemcacheStore

[petergoldstein]

A few years ago we introduced the instrument_errors parameter for DalliStore, that allowed users to configure the store to generate ActiveSupport events in the case of errors. With the removal of DalliStore in favor of MemcacheStore, we need to document what migration path - if any - currently exists.

Resolving this issue may also lead to one or more PRs to the Rails team to add similar functionality to MemcacheStore.