Make error cycling decrement priority for failled computations

[dotsdl]

Credit to @jthorton for bringing this up and offering this solution!

The QCArchive queue is FIFO, so when failed optimizations are error cycled, older submissions jump to the front of the line and can outcompete newer submissions for execution.

As a workaround for this, we can decrement priority for failures from their current value, ending at 'low' priority. This should keep forward progress for well-behaved datasets and computations that haven't had a chance to execute yet.