Handling of struct errors doesn't print the specific error message

[jeffwidman]

When a struct.error is thrown during _pack() or _unpack(), we catch and re-raise as a ValueError: https://github.com/dpkp/kafka-python/blob/master/kafka/protocol/types.py#L11-L12

However, we're shadowing the word error so we lose a handle on the specific exception and cannot print the specific error message.