Pipeline stalls when prefetch can't reach a sample

Example: PRJEB6518 has 534 samples in the database, but some downloads in the prefetch step didn't go according to plan:
```
2023-01-07T04:23:15 prefetch.3.0.1: Current preference is set to retrieve SRA Normalized Format files with full base quality scores.
2023-01-07T04:23:15 prefetch.3.0.1: 295) Downloading 'ERR531601.sralite'...
2023-01-07T04:23:15 prefetch.3.0.1: SRA Lite file is being retrieved, if this is different from your preference, it may be due to current file availability.
2023-01-07T04:23:15 prefetch.3.0.1: Downloading via HTTPS...
2023-01-07T04:23:22 prefetch.3.0.1: HTTPS download succeed
2023-01-07T04:23:22 prefetch.3.0.1: 'ERR531601.sralite' is valid
2023-01-07T04:23:22 prefetch.3.0.1: 295) 'ERR531601.sralite' was downloaded successfully
```
It downloads the `sralite` version because of "current file availability." (This might also happen with 404s, as we've run into previously.) We need a way to:

1) determine how many samples is "enough" to move forward, and more worryingly,
2) convince snakemake to move forward with however many there are.

My first guess for how to tackle this would be to try to extract the names of the failed samples, then rewrite the `SraAccList.txt` file to exclude them. This would be easier for the `sralite` thing above than for the 404s because the "failed" downloads are left behind: When the prefetch job fails, it removes all the downloaded `.sra` files, but it doesn't remove the `.sralite` files that it wasn't expecting to see. So we can look in the project directory for which directories are still there, then **remove those samples from the list.**