Don't treat Kafka connection errors as Makara database errors

[balvig]

We use Kafka alongside Makara in our app, and for some Kafka events we wrap them in a transaction as we want to roll back any changes to the database in case an event doesn't get published.

When event publication fails, a Kafka::DeliveryFailed - Could not connect to any of the seed brokers exception is raised:

ApplicationRecord.transaction do
  record.save
  publish_event #  may raise Kafka::DeliveryFailed - Could not connect to any of the seed brokers:
end

The decription Could not connect to any of the seed brokers however ends up catching this and re-raising it as a Makara::Errors::AllConnectionsBlacklisted exception, due to the line here:

https://github.com/taskrabbit/makara/blob/dac6be2e01e0511db6715b2b4da65a5490e01cba/lib/active_record/connection_adapters/makara_abstract_adapter.rb#L23

...causing some difficulties with the way we track and manage exceptions.

This PR rather naively removes could not connect from the list of CONNECTION_MATCHERS, since all specs still seem to pass, but I'm not sure if there is something I'm unaware of! 🙇

The two errors that could have been affected, are actually caught thanks to the presence of connection refused in their descriptions:

https://github.com/taskrabbit/makara/blob/d609178c3ae278ba377efa2bbdaf10e4f4d71b6a/spec/active_record/connection_adapters/makara_abstract_adapter_error_handling_spec.rb#L37

https://github.com/taskrabbit/makara/blob/d609178c3ae278ba377efa2bbdaf10e4f4d71b6a/spec/active_record/connection_adapters/makara_abstract_adapter_error_handling_spec.rb#L41

Let me know if I've got the wrong end of the stick! 🙏