SCAN.UPC silently fails

### Context

After running [GSE131617](https://www.refine.bio/experiments/GSE131617/genes-associated-with-the-progression-of-neurofibrillary-tangles-in-alzheimers-disease) today there were 2 samples which `failure_reason` was that the file was missing at the time of sha1 generation. This was because SCAN.UPC was not actually writing to the output file since one of the silenced warnings was:

```RRuntimeWarning: /home/user/data_store/raw/TEST/CEL/GSM3791176_Expression_BN_V-VI_13-FC_090612.CEL has a disproportionate number of zero values, so it cannot be processed.```

### Problem or idea

We don't have a good log of why processing this failed as it looks like the file is just missing.

### Solution or next step

We could capture the output of the warnings after `scan_upc` runs then check if the output was generated. If not, throw an exception with the last warning provided from `scan_upc`. This will give us a better way of determining which samples are salvageable.

This would happen at the end of `processors/array_express.py:_run_scan_upc`

Start in processors/array_express.py at _run_scan_upc and inspect how scan_upc warnings are currently silenced and how the output file is checked. Confirm how the warning output and generated-file state are available after processing. Done means a missing output raises an exception containing the last scan_upc warning, improving the failure reason.

Written by the indexing model from the issue text.