pbaylis/econ-program-usage-data: Slight bugfix, reduction of redundant fields.

Resolves 2 issues:

1. Previous version of code was returning FALSE for archives where the code generating these data was unable to read the archive file list. It should have been returning NA.
2. Previous version of code was only using first of multiple possible extensions for each program type.

Neither substantively changes the dataset; few archives are unreadable and the most common extension was usually listed first anyway.