Error node needs extension from Maybe to Either E

[rokopt]

The new error node (see #90 and #129) currently introduces Maybes for places where it passes errors back to clients (see also #132, which proposes an extension to allow clients to choose to crash right away rather than catch and return errors). It will need to be extended to Either E, where E is a client-chosen type. This subsumes not only Maybe (where E is Unit) but also circuits where there are no failures (where E is Void), thus undoing the regression that we can no longer represent circuits that can be statically guaranteed not to fail, as well as allowing clients to distinguish among different, configurable classes of errors.