For per-document accessors generation, catch ValueError exceptions

when applying accessors from properties and only display a message in
the logs.

This exception is raised within the property documents when a property
is invalid (invalid type, category name, reference...)


git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@43887 20353a03-c40f-0410-a6d1-a30d3c3de9de