[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.

bgcolor = '#000000'
stylesheets = ['%s/cubicweb.css' % datadir_url]
logo = '%s/logo.png' % datadir_url
lazy = lazystr('%(bgcolor)s')