Guard against (currently impossible) failure to read from `documentStore`.
Guard against (currently impossible) failure to read from `documentStore`.

While the implementation of the `documentStore` is currently simple enough
to never throw (it is in-memory), it makes some sense to guard against future
extended functionality where an exception might be raised.

Since storing this object in a distributed memory store isn't currently
feasible, I'm not sure what such an exception would be right now, but I
don't mind being proactive!

Ref: https://github.com/apollographql/apollo-server/pull/2111/files#r247618501

apollostack/apollo-server