Before this change conversion tasks would be queued for assignments that
did not have the pdf annotation method enabled.
The change will also prevent queing when the plugin is disabled.
Without this, its possible to get a document in the editpdf conversion queue
that keeps failing - and in some cases completely crashing the PHP script
despite exception handling - which ends up blocking the processing of the queue
entirely.
This change allows for a configurable number of attempts the conversion
task will perform before removing the item from the queue.
Neill Magill