Contents insertion on Lookup table should fail gracefully

[eywalker]

trafficstars

Currently if someone is using a dj.Lookup table with contents field that has more entries then it is found in the database but if the person doesn't have insert and update privilege, the table definition fails. It should be that if insert/update fails, then it should only issue a warning but not prevent the whole import.

[guzman-raphael]

Read-only users also discussed in #636