[dataimport] backout 6947201033be (related to #2788402)
(and add a try: except to cache the intended error)
The problem actually comes from the ``MassiveObjectStore``
in the ``dataio`` cube, so it should be corrected there.
Here, we only protect it with a ``RuntimeWarning`` so that
the user can see the problem.
``value`` is set to ``None`` (whence to ``NULL`` from
a database standpoint), so that the data can be nevertheless
inserted in the database. However, only the keys present in
``row`` are actually non-'``NULL``'.
The real solution is to work out the issue in ``MassiveObjectStore``
directly.
The current try/except should only be a temporary hack.
