Add. set_logger function allow logging errors (crash Lua VM) in current llthread's threads.

``` Lua
local llthreads = require "llthreads"
local LOG = require"log".new(
  require "log.writer.net.zmq".new("tcp://127.0.0.1:5555")
)
llthread.set_logger(function(msg) LOG.error(msg) end)
```