cubicweb #3740864 Debug of the `_return_to_lastpage` [in-progress]

add a info on the _return_to_lastpage method explaning the origin of the return url in the case the '__redirectpath' is not defined in self._cw.form.

prioritynormal
typeenhancement
done in<not specified>
load0.100
load left0.100
closed by<not specified>
patch[traceability] add a useful debugging info on the `_return_to_lastpage` method (closes #3740864) [rejected]