Allow parsers to manually return errors

[ObserverOfTime]

Problem

HTML and XML need to return error nodes when finding mismatched tag names. Parsers like readline need to prevent errors from affecting other lines. Aliasing nodes to $.ERROR does not construct an actual error symbol under the hood.

Expected behavior

This could either be implemented as a special case to alias() where alias($.foo, $.ERROR) wraps $.foo in an error symbol, or as a new function error([node]) which will wrap the node or directly return an error if called without one.