Bug 1615022 part 1. Allow move-constructing ErrorResult from CopyableErrorResult. r=farre

We need the explicit constructor because otherwise which conversion operator to
use when constructing an ErrorResult from a CopyableErrorResult&& is
ambiguous. And we want the conversion operator to allow passing
CopyableErrorResult& to functions that expect ErrorResult&&.

Differential Revision: https://phabricator.services.mozilla.com/D62631