Use TTG empty extensions to remove some `error`'s

None of these error cases should ever have been reachable, so this is
just a matter of leveraging the type system to assert this.

  * Use the `NoExtCon` and `noExtCon` to handle case matches for no
    extension constructors, instead of throwing an `error`.

  * Use the extension field of `HsSpliceTy` to ensure that this variant
    of `HsType` cannot exist in an `HsType DocNameI`.

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