Fix to work with epoch, and improve errors

This fixes `new IntlRelativeFormat().format(0)` so that it correctly
compares "now" with the epoch.

These changes also improve the error messages, and make most of them of
type `RangeError`.

Fixes #5
Add back language tag validation
Switch most RangeErrors back to Error
Merge pull request #6 from ericf/epoch

Fix to work with epoch, and improve errors
Fix CLDR plural rule functions data

This fixes the CLDR plural rule functions data and rebuilds all the
locale data for the locales that were fixed.

Fixes #10
Merge pull request #12 from ericf/fix-cldr-plural-data

Fix CLDR plural rule functions data