Minor bug fixes

[dfaller]

Here are a couple minor bug fixes we need:

1. Update the JobExecution model to check for strings in the configuration and resources JSON fields. When the column was updated from json to jsonb, it appears some values come back as strings instead of dicts in Django.

2. Update ProductFileManager.populate_source_ancestors() to be more efficient. We should remove the file_type == 'SOURCE' condition from the query and do that filtering ourselves because that filtering in the database causes the query to perform very poorly.

3. Correct exception handling to the Scale Database Update task.