Ambigous error when consumer is fenced by group.instance.id

Description
===========
When using the `group.instance.id` for two consumers and they conflict, the fenced client receives an ambiguous error.

The `.code()` of the error is `KafkaError._FATAL` which is generic, but the `.str()` of the error suggests that is specifically a `KafkaError.FENCED_INSTANCE_ID`.
Strangely the `.str()` of the error is duplicated/redundant for some reason.
Here is the ambiguous error exactly: `KafkaError{code=_FATAL,val=-150,str="Fatal error: Broker: Static consumer fenced by other consumer with same group.instance.id: Fatal consumer error: Broker: Static consumer fenced by other consumer with same group.instance.id"}`.

Another possible issue with the error is that the `.code()` suggests this is an unrecoverable error, but the `.fatal()` returns false.

How to reproduce
================
Replace the `kafka_servers` with your development servers and set `topic` to any valid and empty topic.
The last assertion fails because the error is ambiguous.
```
topic = "testingtopic"
kafka_servers = ["127.0.0.1:9092"]

def _new_consumer():
return Consumer(
{
"bootstrap.servers": " ".join(kafka_servers),
"group.id": "my_fleet",
"group.instance.id": "ship_0",
"enable.auto.commit": False,
"enable.auto.offset.store": False,
"default.topic.config": {"auto.offset.reset": "earliest",},
}
)

# start first consumer instance
first_c = _new_consumer()
first_c.subscribe([topic])
assert first_c.poll(timeout=10) is None
assert len(first_c.assignment()) > 0

# start replacement consumer instance (first will be fenced off in favor of this)
second_c = _new_consumer()
second_c.subscribe([topic])
assert second_c.poll(timeout=10) is None
assert len(second_c.assignment()) > 0

# check if first consumer is fenced off as it should be
msg = first_c.poll(timeout=5)
assert msg is not None
assert msg.error() is not None
assert msg.error().code() == KafkaError.FENCED_INSTANCE_ID, "%s %s" % (msg.error().fatal(), msg.error(),)
```

Checklist
=========
Please provide the following information:

- [x] confluent-kafka-python and librdkafka version: `('1.4.1', 17039616), ('1.4.0', 17039615)`
- [x] Apache Kafka broker version: `2.5.0 (bitnami docker image)`
- [x] Client configuration: `included in the reproduction snippet`
- [x] Operating system: `Arch Linux (Linux lgg 5.6.5-arch3-1 #1 SMP PREEMPT Sun, 19 Apr 2020 13:14:25 +0000 x86_64 GNU/Linux)`
- [ ] Provide client logs
- [ ] Provide broker log excerpts
- [ ] Critical issue

I'm not sure what to provide with client/broker logs as everything works, except the error seems to be badly constructed. If more information is needed I'll provide it.