Add more descriptive error message when encountering UnknownMemberIdError while advancing offsets

[lennrt]

Problem

If an UnknownMemberIdError is encountered while using kafka-consumer-manager offset_advance, users will see an error of the following form:

kafka.errors.UnknownMemberIdError: [Error 25] UnknownMemberIdError: OffsetCommitResponsePayload(topic='some_kafka_topic', partition=0, error=25)

This can be a bit opaque for users who are not familiar with the internals of Kafka.

Solution

We give users a more practical error message which instructs them to stop their consumer prior to attempting to advance offsets.

Verification

tests pass

Release Plan

patch release