utils/converters: use ValidationError instead of Exception

This ensures that after validation the page is resolved as a 404 instead
of displaying a 500.

app/utils/converters.py

@@ -63,7 +63,7 @@ class PageConverter(BaseConverter):
 
         t = self.object.query.filter_by(id=tid).first()
         if t is None:
-            raise Exception(f"BaseConverter: no object with id {url}")
+            raise ValidationError(f"BaseConverter: no object with id {url}")
 
         return (t, page)