[Refactor] make NodeError exhaustive

[alexmozaidze]

Non-exhaustive tag on an error enum means that the user is forced to add a wildcard saying "if I didn't account for other errors then do X", where "X" is most likely a runtime panic, which is not ideal, especially in cases where the user actually wants the compiler to ensure that he covered all possible cases of failure.

Non-exhaustive enums are mainly for configuration enums and such, where items are likely to be added and a missing case is no big deal, which does not apply to errors.