[AC-549] Introduce failed_data_rows to the Task class.

[jakubborowski-lb]

• In order to support a backend change that is going to allow data row imports to succeed even when a small chunk of data rows might not get created, we're introducing the notion of failed_data_rows. These data row inputs will be returned in the results file on an import task.
• Remove unused imports.

Example output of a result file:

{
  "createdDataRows": [
    {
      "id": "cl8kui84n019hatrw5c17gdcl",
      "rowData": "https://storage.googleapis.com/image.jpeg",
      "datasetId": "cl8kui65h00bxatrwc2jifig0"
    },
    // additional created rows...
  ],
  "errors": [
    {
      "message": "Failed to create data rows after 5 retries. This chunk's data rows will not be imported.",
      "failedDataRows": [
        {
          "rowData": "https://storage.googleapis.com/image.jpeg"
        },
        // additional failed rows...
      ]
    },
    {
      "message": "Failed to create data rows after 5 retries. This chunk's data rows will not be imported.",
      "failedDataRows": [
        {
          "rowData": "https://storage.googleapis.com/image.jpeg"
        },
        // additional failed rows... 
      ]
    }
  ]
}

[kkim-labelbox]

LGTM!