When using `safe_transmit`, wrap unexpected exceptions that are not alre...

...ady wrapped in `Beaneater::UnexpectedResponse` and raise the wrapped exception.

This allows applications using beaneater to catch these exceptions and
act accordingly instead of raising exceptions comming from beaneater.

We noticed this problem when shutting down beanstalk. beaneater raises
a `NoMethodError` instead of an exception that can be easily rescued
in our system.