Fix field name that the API actually returns

[c-mart]

When querying /usage/failed, this Python client looks for a key named ValidationFailedRecords in the response object. But the API actually returns a key named FailedRecords:

$ curl -s -H "XA-SITE: IU" -H "XA-API-KEY: redacted" https://usage.xsede.org/api/v1_test/usage/failed | jq -r . | head -n 6
{
  "FailedRecords": [
    {
      "Charge": 166.158,
      "EndTime": "2022-09-14T00:00:00Z",
      "Resource": "jetstream2.indiana.xsede.org",

As a result, client.get_failed_records() was returning <FailedUsageResponse: 0 records> when we definitely had some failed records. With the change in this PR, I now see <FailedUsageResponse: 1539 records>, what I would expect.