[webcomponents] The algorithm "invoke callbacks" must halt as soon as one of callbacks throw an exception (#421)

The algorithm to [invoke callbacks](https://w3c.github.io/webcomponents/spec/custom/#dfn-invoke-callbacks-in-element-queue) should halt as soon as one of these callbacks throw an exception, and re-throw that exception.  Otherwise the whole idea of re-throwing exceptions from DOM API is non-sensical.

This has an interesting implication that if there were multiple unresolved elements queued up for upgrades and one of constructor calls throws an exception, then all subsequent elements would never be upgraded. 

---
Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/421