Fix failure to start with uninitalised database

Fix failure to start with uninitalised database

An OptionalModelChoiceField will attempt to query the database to get choices
in its __init__ method. This fails if the database hasn't been initialised
yet. So, put that in a try/catch block. This lets things work through the
migration and loading of data from fixtures.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Reviewed-by: Stephen Finucane <stephenfinucane@hotmail.com>