Fix traceback in gears.protected_call (#1178)

The traceback should not include the error handler because this is confusing.

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

lib/gears/protected_call.lua

@@ -13,7 +13,7 @@ local xpcall = xpcall
 local protected_call = {}
 
 local function error_handler(err)
-    gdebug.print_error(traceback("Error during a protected call: " .. tostring(err)))
+    gdebug.print_error(traceback("Error during a protected call: " .. tostring(err), 2))
 end
 
 local function handle_result(success, ...)