No warnings if nothing is created.

[GeoWill]

If all first_letter_to_outcode_parquet fail, (eg because the source_path is misconfigured, so they think there's nothing there) then they ll complete, but no files are written.

If we have no files, this is obvious, because we still have no files, and we would expect to have some files. This is harder to spot when we already have some files.

Possible fixes:

• output the number of parquet files written, and count them all and check it hits a threshold.
• look at the modified time on the s3 files we're expecting.