`UnsupportedVersionError` is intended to indicate a server-side error:
https://github.com/dpkp/kafka-python/blob/ba7372e44ffa1ee49fb4d5efbd67534393e944db/kafka/errors.py#L375-L378

So we should not be raising it for client-side errors. I realize that
semantically this seems like the appropriate error to raise. However,
this is confusing when debugging... for a real-life example, see
https://github.com/Parsely/pykafka/issues/697. So I strongly feel that
server-side errors should be kept separate from client-side errors,
even if all the client is doing is proactively protecting against
hitting a situation where the broker would return this error.

In Python3, `ConnectionError` is a native exception. So rename our
custom one to `KafkaConnectionError` to prevent accidentally
shadowing the native one.

Note that there are still valid uses of `ConnectionError` in this code.
They already expect a native Python3 `ConnectionError`, and also already
handle the Python2 compatibility issues.