hashes: Fix leaf error type

Leaf error types should typically have private fields, provide accessor
functions, and not use `non_exhaustive`.

hashes/src/lib.rs

@@ -234,12 +234,19 @@ pub trait Hash:
 
 /// Attempted to create a hash from an invalid length slice.
 #[derive(Debug, Clone, PartialEq, Eq)]
-#[non_exhaustive]
 pub struct FromSliceError {
     expected: usize,
     got: usize,
 }
 
+impl FromSliceError {
+    /// Returns the expected slice length.
+    pub fn expected_length(&self) -> usize { self.expected }
+
+    /// Returns the invalid slice length.
+    pub fn invalid_length(&self) -> usize { self.got }
+}
+
 impl fmt::Display for FromSliceError {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         write!(f, "invalid slice length {} (expected {})", self.got, self.expected)
@@ -247,9 +254,7 @@ impl fmt::Display for FromSliceError {
 }
 
 #[cfg(feature = "std")]
-impl std::error::Error for FromSliceError {
+impl std::error::Error for FromSliceError {}
-    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> { None }
-}
 
 #[cfg(test)]
 mod tests {