Fix UDF date formatting

Fixes #1046
@RickMohr @jwalgran You're right, UDF audits are deleted when UDFs are deleted (https://github.com/OpenTreeMap/OTM2/blob/3e4de471ba9abc8dd0f9a694f5ebc585882e5045/opentreemap/treemap/udf.py#L531-L565)

So, I'll go with Justin's suggestion of throwing a helpful error message here.