Check FANTOIR filename using FANTOIRMMYY format

Summary:
Similar to fc13adda27ce, we need to check for FANTOIRMMYY format,
ie for FANTOIR0423 and not for FANTOIR423.

The zerofill is added to the year to so it works with older files too.

Test Plan:
Run fantoir-datasource fetch twice:

• first time should exit with 0, FANTOIR0423 should exist
• second time should exit with 1Z and warn file already exists

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D3285