Make i18n:generate more flexible

Now normal i18n:generate doesn't care about missing files, and
i18n:generate_strict does.  The robot task calls the strict version.

There may well be a better way to make the two tasks, but my Ruby+Rake
skillz are weak, and it's all going away soon.
Merge pull request #2485 from edx/ned/make-i18n-generate-forgiving

Make i18n:generate more flexible