Support re-queueing Failed tasks

[Metamess]

Issue #20 is a generalized variant of this feature, though that focusses on changing the phase of individual tasks. This feature request would include an easy way to move all tasks in "failed" back into "incoming" in a single command.

Optionally, combine with #19 to allow the re-queued tasks to be inserted at the head of the queue, instead of the tail