nvim: show errors in handler on stderr when used as client

[bfredl]

fixes #216. A modified test case to cover both cases:

import sys
import os
from neovim import attach

def setup_cb():
    nvim.command('call rpcnotify({}, "foo")'.format(nvim.channel_id))
    nvim.call('rpcrequest', nvim.channel_id, "bar", "xx", async=True)


def notification_cb(msg, _args):
    num -= 1

def request_cb(name, args):
    FAIL


nvim = attach('child', argv=['nvim', '-u', 'NONE', '--embed'])
nvim.run_loop(request_cb, notification_cb, setup_cb)

This should not affect plugins running inside vim, as the plugin host already catches Exception:s and displays the plugin traceback appropriately.