net.box: fix IO error messages

1. The net.box state machine is supposed to set E_NO_CONNECTION on
   error, but netbox_transport_connect returns an error returned by
   coio_connect and coio_read as is. Fix this.

2. netbox_transport_communicate sets the error message to
   strerror(errno) on coio_read or coio_write error. This works now,
   because iostream can only be backed by plain sockets, but once we
   implement encryption, errno won't make any sense. We should use
   error::errmsg instead.

3. While we are at it, fix luaT_netbox_transport_make_request: it's
   wrong to pass errmsg directly to box_error_raise - we should use a
   format specifier (%s).

Fixes commit c13b3a31 ("net.box: rewrite
state machine (transport) in C").