Handle Propel exception in filter, refs #9533
Handle Propel exception in filter, refs #9533

We need to handle the possibility of Propel not delivering a connection where
there is none available, e.g. in the AtoM installer before the database is
configured (missing `config/config.php`). There might be other scenarios where
this is possible too and we want to handle it gracefully.

artefactual/atom,artefactual/atom,artefactual/atom