Additonal validation for hard-coded era data
Additonal validation for hard-coded era data

This adds an additional check to make sure that the (hard-coded)
era definitions for each calendar aren't missing required `isoEpoch`
properties. Note that this code is only run once at load time, not each
time calendars are accessed, so there's no runtime impact.

js-temporal/temporal-polyfill,js-temporal/temporal-polyfill,js-temporal/temporal-polyfill