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