Add unused variants to ParseError and SyntaxViolation enums

Per discussion in https://github.com/servo/rust-url/issues/498#issuecomment-513299432 , this adds a hidden unused variant to each of `ParseError` and `SyntaxViolation` enums so as to make future additions non-breaking changes.   This is the long standing manual way (e.g. originally used by`std::io::ErrorKind`), given lack of a stable feature and acceptable rust MSRV with rust-lang/rust#44109.

Also these types already implemented `Display` but having been burnt with `Debug` vs `Display` versions of the `fmt` function, I also disambiguated paths in these `Display` implementations.

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