rpc: pass errors from async_accept
rpc: pass errors from async_accept

According to the [boost::asio documentation](http://www.boost.org/doc/libs/1_55_0/doc/html/boost_asio/reference/basic_socket_acceptor/async_accept/overload2.html),
the function signature of the handler must be:

    void handler(
      const boost::system::error_code& error // Result of operation.
    );

We were binding *all* the arguments, instead of all but the error,
resulting in nullary function that never got the error. Fix this
by adding an input argument substitution.

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