bigquery: Batch load example does not handle errors correctly

[JurjenBiewenga]

Currently the Batch load example just logs the AppendResult, but ignores the Error/RowErrors fields. This makes the log look like it succeeded and there were no issues appending your rows. Proposing that the example should be checking the Error field, if there's content, then log all errors relating to the appended rows. E.g. something like:

if (responseItem.Error != null)
{
    await Console.Error.WriteLineAsync($"Appending rows resulted in an error: {responseItem.Error.Message}");
    foreach (RowError rowError in responseItem.RowErrors)
        await Console.Error.WriteLineAsync(rowError.ToString());
}