Describe the bug
Consumers and Producers when __aenter__() errors (such as network-related exceptions) leaves the object in a state requiring an explicit close() call.

Context manager interfaces via the "with" keyword do not invoke __aclose__() if entering failed.

Expected behaviour
__aenter__() and start() if an exception raises put the object back into an "unentered" state, and potentially allow trying again later.

Environment (please complete the following information):

• aiokafka version (python -c "import aiokafka; print(aiokafka.__version__)"): 0.12.0
• Kafka Broker version (kafka-topics.sh --version): 2.8.1

Reproducible example
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