Add error handling for missing/misordered reference tables

It's super easy to add a reference table to the schema and then neglect to add it to the reference_table list. The order in which the reference tables are listed is also important. It would be good to have some error handling setup specifically for these two cases.

Start by locating the schema definition and the reference_table list, then trace where reference tables are loaded or validated. Add clear errors for a schema table missing from the list and for an invalid table order; done means both cases are detected consistently.

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