Read a template file without importing it

What the file declares: the template it is, the columns it needs, whether it carries example data, whether it carries the template's card image, and — the reason this is a separate call — whether the catalog already has it. Uploading a template that already exists updates it in place rather than adding a second copy, and someone has to be told that before they commit rather than after. A file carrying NO data also comes back with dataset_names — this workspace's datasets, so the name it has to be bound to can be completed rather than guessed. Writes nothing.