Opening this as a follow-up to #924... because it is slightly different, and more complicated to fix.

As I was debugging #924 I came across this comment in our code for loading and displaying migration messages (eg: validation errors) after a CSV import:

This made me realize that this code will not work under certain conditions. Specifically, if this is the first time that a certain CSV importer is used, and validation fails, then the migrate_map_* table will not be created, and we will not be able to load the validation message.

Steps to reproduce:

1. Install farm_migrate_csv on a fresh farmOS instance.
2. Download the template for an Activity log CSV.
3. Only fill in the name column of the first row (leave timestamp blank so that validation fails).
4. Import the CSV.
5. Notice that the validation message is not shown.

This is a bit of a deep issue because the way we are loading the messages (after migration happens) relies on the migrate_map_* table existing, but Drupal core doesn't create that when no items are imported.