Implement std::error::Error::source for MerkleBlockError

The `MerkleBlockError` type does not implement `Display` or
`std::error::Error` - bad rust-bitcoin developers, no biscuit.

src/util/merkleblock.rs

@@ -40,6 +40,8 @@
 //! assert_eq!(1, index[0]);
 //! ```
 
+use core::fmt;
+
 use crate::prelude::*;
 
 use crate::io;
@@ -53,20 +55,45 @@ use crate::consensus::encode::{self, Decodable, Encodable};
 use crate::util::merkleblock::MerkleBlockError::*;
 use crate::{Block, BlockHeader};
 
-/// An error when verifying the merkle block
+/// An error when verifying the merkle block.
 #[derive(Clone, PartialEq, Eq, Debug)]
 #[non_exhaustive]
 pub enum MerkleBlockError {
-    /// When header merkle root don't match to the root calculated from the partial merkle tree
+    /// Merkle root in the header doesn't match to the root calculated from partial merkle tree.
     MerkleRootMismatch,
-    /// When partial merkle tree contains no transactions
+    /// Partial merkle tree contains no transactions.
     NoTransactions,
-    /// When there are too many transactions
+    /// There are too many transactions.
     TooManyTransactions,
-    /// General format error
+    /// General format error.
     BadFormat(String),
 }
 
+impl fmt::Display for MerkleBlockError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        use self::MerkleBlockError::*;
+
+        match *self {
+            MerkleRootMismatch => write!(f, "merkle header root doesn't match to the root calculated from the partial merkle tree"),
+            NoTransactions => write!(f, "partial merkle tree contains no transactions"),
+            TooManyTransactions => write!(f, "too many transactions"),
+            BadFormat(ref s) => write!(f, "general format error: {}", s),
+        }
+    }
+}
+
+#[cfg(feature = "std")]
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
+impl std::error::Error for MerkleBlockError {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        use self::MerkleBlockError::*;
+
+        match *self {
+            MerkleRootMismatch | NoTransactions | TooManyTransactions | BadFormat(_) => None,
+        }
+    }
+}
+
 /// Data structure that represents a partial merkle tree.
 ///
 /// It represents a subset of the txid's of a known block, in a way that