error is nill on result of save when 409 returned from arangodb

Case needed in
func (col *Collection) Save(doc interface{}) error {
...
case 409:
return errors.New( "unique constraint violated" )

To reproduce, add a unique index on a collection.
Call to add a document multiple times, the error is returned by Arango but not by the save function.

I believe this should be added to other functions as well.