Errors in callbacks don't terminate the loop

As the corocbk.rethrow test shows, errors that happen directly inside an idle source stop the current loop and are returned by loop:run().

However, when the error occurs somewhere nested inside callbacks, the callback chain is aborted, but the error is swallowed.
In the following minimal example, if the query_info operation fails, the error will be printed as GLib warning, but the loop will not quit immediately.
Only once the timeout hits will the loop stop, but report a "successful" true, nil.

The Guide suggests that these errors should either terminate execution or trigger a pcall, neither of which happens here.

local lgi = require("lgi")
local Gio = lgi.Gio
local GLib = lgi.GLib
local File = Gio.File
local loop = GLib.MainLoop()

local co = coroutine.create(function()
    local f = File.new_for_path("./does_not.exist")
    local ok, err = pcall(f.query_info_async, f, "standard::type", 0, GLib.PRIORITY_DEFAULT, nil, function(_, token)
        local _, err = f:query_info_finish(token)
        assert(err == nil)
        loop:quit()
    end)
    print(ok, err)
end)

GLib.idle_add(GLib.PRIORITY_DEFAULT, co)
GLib.timeout_add(GLib.PRIORITY_DEFAULT, 1000, function()
    loop:quit()
end)

local ok, err = pcall(loop.run, loop)
print(ok, err)

Result:

true    nil

(process:17302): Lgi-WARNING **: 11:46:45.041: Error raised while calling 'lgi.cbk (function: 0x55e197403ea0): Gio': test.lua:11: assertion failed!
true    nil

Related: #124.

Start with the corocbk.rethrow test in tests/corocbk.lua and compare its direct idle-source behavior with the nested callback example in the issue. Read the callbacks section in docs/guide.md and trace loop:run through the callback chain. Done means errors from nested callbacks are not swallowed and loop:run reports the failure instead of true, nil.