Don't error if no files found to import

Easier to see diff with https://github.com/mysociety/sayit/pull/409/files?w=1

This is especially relevant with #387. For OpenHouseNS, for example, we load an entire directory hourly with the --no-clobber option. Another script adds new AN files to that directory. Most hours there's nothing to load. Currently, we get emails because the command failed to run. However, not having anything to load is not an error state; not having anything to load is just informational.