Hard failures for SYNQueueTask #3
Description
Current behavior is that a task is retried if it fails, up till the max retry count.
Some scenarios might require a "hard fail" which means "this task failed, don't retry".
One approach might be to create an error code (kSYNQueueErrorCodeHardFail) that can be used to identify hard failures. The task handler will call task.completed(error) with the hard fail code when necessary. The queue will have to check every failure for this error code, and if it exists, instead of retrying, just cancel the task.