fix error when >1 draft SO are checked

when the wizard would run, then the @api.multi method `detect_exceptions` would
fail because it returns an attribute but it is called on a recordset of size >1

Code refactoring to:

* move the side effect of writing to so.exception_ids one method up
* document the side effect in the docstring
* fix the computation of the return value for the `detect_exceptions` method