Conversion will never be retried if a transient failure is encountered during `are_requirements_met` check

[cameron1729]

Currently there is some code in poll_conversion_status to check various statuses from S3 and, if appropriate, keep the conversion in progress so it can be retried.

However, if a transient failure happens during the are_requirements_met check, and there are no other converters available, the conversion will be marked as failed by core (see also get_document_converter_classes) and the conversion will never be attempted again even though the transient failure may have resolved.