More informative message for existing resource_ids
Fix warning message for existing resource_ids.

- It now uses, more correctly, 'self.resource_id';
- It also shows the '__repr__()' of the referred object.
Merge branch 'master' into res_id
Fix __repr__() for Event() class.
Merge remote branch 'claudiodsf/res_id' into res_id
Forcing the use of the inherited __str__ method for the __repr__ method of the Event() class
Merge pull request #669 from claudiodsf/res_id

More informative message for existing resource_ids