fix error with >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
fix flake8 warning
fix bad use of api.one
Merge pull request #38 from gurneyalex/8.0_fix_sale_exceptions

fix error when >1 draft SO are checked