Add non_exhaustive to all error types

As we are doing in `rust-bitcoin`; to enhance forward compatability, and
ease the upgrade path for downstream projects, add `non_exhaustive` to
all error types.

Author: tcharding

src/error.rs

@@ -19,6 +19,7 @@ use core::fmt;
 
 /// Crate error type.
 #[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+#[non_exhaustive]
 pub enum Error {
     /// Tried to create a fixed-length hash from a slice with the wrong size (expected, got).
     InvalidLength(usize, usize),

src/hex.rs

@@ -30,6 +30,7 @@ use crate::Hash;
 
 /// Hex decoding error.
 #[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+#[non_exhaustive]
 pub enum Error {
     /// Non-hexadecimal character.
     InvalidChar(u8),