Gracefully handle error on prettify_dt

prettify_dt now returns the empty string '' if we try to call it
on a non date.

This situation occasionally happens on development DB.

NOTE: I'm not certain this is the correct behaviour.  Might be better to `croak` or confess here, to get more diagnostics?  (Either way, *some* code is needed here, to avoid simply dying with a confusing error in perllib/Utils.pm)  What do you think?