Explicitly check HTTP status of data.http resources

[tgeoghegan]

In #2088, we deleted some lifecycle.postconditions blocks on data.http resources in Terraform module data_share_processor because they caused problems with planning when adding new localities. Failure to fetch the JSON manifests, or unexpected content, should still cause obvious failures, but ideally we should have explicit checks.

Perhaps this is due to a bug in Terraform that will be fixed, in which case we should put those postconditions declarations back. In the worst case, we can do checks on data.http.<resource>.result.http_code, though Terraform lacks anything like an assert statement.