Ported from tree-sitter/haskell-tree-sitter#149.

We could simplify this logic (and the similar logic over :+:s in the generic counterpart) by changing showFailure to take Maybe Node instead of Node. Then we could do fail $ showFailure (Proxy @(Either a b)) currentNode on this line, and on the line above, we wouldn’t need to return node as part of the triple since the later calls to showFailure could use the original binding of currentNode :: Maybe Node from the first line of the do-block.