Ignore JetBrains path if not found

Summary:
If the script runs on a machine without JetBrains IDE installed,
like a remote dev server with hunspell (or nothing), it currently
fails loudly. Instead, it should skip the import.

Fixes the following issue:

FileNotFoundError: [Errno 2] No such file or directory:
'/home/dereckson/.config/JetBrains'

Test Plan:
bin/merge-dictionaries --extract on a machine
where any JetBrains IDE isn't installed.

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2743