Bug 1619112 part 5. Stop accepting nsAString in ErrorResult error-throwing. r=smaug

Callers should pass in UTF-8, since that's what the JS engine ends up with in the end anyway.

The various URL changes are because NS_NewURI converts incoming nsAString to
UTF-8 anyway. So we might as well do that up-front and then use the UTF-8
string for both the NS_NewURI call and the error-reporting if it fails.

Differential Revision: https://phabricator.services.mozilla.com/D65543