no strack trace in callback errors

Currently when an error occurs inside a callback, it displays the error message via a gtk warning, but it doesn't display a proper stack trace like a normal Lua error. This makes debugging problems inside callbacks hard.

Example code:

local lgi = require "lgi"
local Gtk = lgi.Gtk

local function evil_code(w)
    w:add(nil)
end

local function some_stuff(w)
    evil_code(w)
end

local window = Gtk.Window{
    on_destroy = Gtk.main_quit,
    Gtk.Button{
        label = "Click to crash",
        margin = 10,
        on_clicked = function(w)
            some_stuff(w)
        end
    }
}

window:show_all()
Gtk.main()

example output after causing an error:

(lua:4090): Lgi-WARNING **: Error raised while calling 'lgi.cbk (function: 0x1001410): GObject': /usr/local/share/lua/5.1/lgi/override/Gtk.lua:139: bad argument #3 to 'add' (Gtk.Widget expected, got nil)

expected output: a full stack trace similar to this

lua: ./error_test.lua:6: attempt to index local 'w' (a nil value)
stack traceback:
    ./error_test.lua:6: in function 'evil_code'
    ./error_test.lua:10: in function 'some_stuff'
    ./error_test.lua:14: in function <./error_test.lua:13>
    ./error_test.lua:15: in main chunk

Using lgi 0.9.0 under Gentoo Linux x86_64, with Gtk 3.16.7

Reproduce the callback failure using the Lua/Gtk example and the stated lgi 0.9.0 environment, then trace how callback errors are reported. Done means the warning includes a full Lua stack traceback showing the callback's calling functions, rather than only the error message.