Make `ParseError` exhaustive

[alexmozaidze]

Make ParseError exhaustive. Having it non-exhaustive means that one must add a wild-card to checking for errors, even if it is required to account for all the errors explicitly (having a runtime _ => unreachable!() is not very good). Making it a normal enum allows the user of the library to decide for themselves whether or not they want to wild-card match other cases.

[valenting]

I believe that's intentional, to future proof that enum.
If it were exhaustive, then consumers must check all errors, so it's impossible to add another ParseError item without breaking crates that are using it.