[migration] fix bug in `CWAttributeAddOp.revertprecommit_event`
The `rdefdef` attribute is set during the operation execution. In some the
operation crash before this assignment. In such case `revertprecommit` raise an
attribute error crashing the whole process and shadowing the original error.
This changeset detect and Avoid this situation.
# Run all scenarii found in windmill directoryfromos.pathimportjoin,dirnamefromcubicweb.devtools.cwwindmillimport(CubicWebWindmillUseCase,unittest_main)classCubicWebWindmillUseCase(CubicWebWindmillUseCase):#test_dir = join(dirname(__file__), "windmill/test_edit_relation.py")passif__name__=='__main__':unittest_main()