gears.timer: Fix the traceback on errors

With the second argument being 2, the traceback will not include the error
handling function, but instead end at the actual place of the error.

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/gears/timer.lua

@@ -109,7 +109,7 @@ capi.awesome.connect_signal("refresh", function()
         local success, message = xpcall(function()
                 callback[1](unpack(callback, 2))
             end, function(err)
-                print(debug.traceback("Error during delayed call: "..tostring(err)))
+                print(debug.traceback("Error during delayed call: "..tostring(err), 2))
             end)
     end
     delayed_calls = {}