Problem
When the `moreToCome` flag is set in a request, the receiver must not send a response back based on these docs: https://www.mongodb.com/docs/manual/reference/mongodb-wire-protocol/#flag-bits.

The injector is a binary that responds mongod requests (more info here). When the injector incorrectly sent a response back for a killCursors request with `moreToCome` set, mongod logged a confusing error: "BSON field 'CursorResponse.cursor' is missing but a required field". I think it logged this because it expected a find/getMore response (which requires a cursor field).

Solution
We should improve the error message to be something like "got an unexpected response for a request with the `moreToCome` flag set" when this protocol is violated.

Impact
Clearly describe when the protocol is violated. Help future debugging with injector <--> mongod interactions.