Merge rust-bitcoin/rust-bitcoin#1139: Do error cleanups

abc014a079 Move rustdoc above attribute (Tobin C. Harding)
e9230019eb Implement std::error::Error::source for MerkleBlockError (Tobin C. Harding)
613f1cf2d5 Implement std::error::Error::source for bip152 (Tobin C. Harding)
a37ab82503 Add non_exhaustive to _all_ errors (Tobin C. Harding)

Pull request description:

  Do error cleanups, and add a script to help find missing `non_exhaustive` on error types

  - Patch 1: Audit the codebase and put `non_exhaustive` on all  error types.
  - Patch 2: Implement `std::error::Error::source` for `bip152::Error`
  - Patch 3: Implement `Display` and `std::error::Error::source` for `MerkleBlockError`
  - Patch 4: Move rustdocs to above attributes on one error type
  - ~Patch 5: Add a python script to `contrib` that checks the codebase for missing non exhaustive on the line above the results of regex `pub enum .*Error`~

  I removed the Python script patch, I can't be bothered working on this for now but the clean ups and `non_exhaustive` additions are useful IMO.

  ### labels
  - I added 'API break', I think the `non_exhaustive` addition requires major bump but not sure?
  - release notes mention is just for the `non_exhaustive` patch.

ACKs for top commit:
  Kixunil:
    ACK abc014a079
  apoelstra:
    ACK abc014a079

Tree-SHA512: 16ea15014eae97de7ac9cca1e9b76304aa3702a98cde577c2d71343022f840d3b33a39d2ab6d3fba0f0f1ebaa1631a0595eea1d794ba9727fe6ccfcf08e2feae

src/blockdata/transaction.rs

@@ -250,9 +250,9 @@ impl Default for TxIn {
 #[cfg_attr(feature = "serde", serde(crate = "actual_serde"))]
 pub struct Sequence(pub u32);
 
+/// An error in creating relative lock-times.
 #[derive(Clone, PartialEq, Eq, Debug)]
 #[non_exhaustive]
-/// An error in creating relative lock-times.
 pub enum RelativeLockTimeError {
     /// The input was too large
     IntegerOverflow(u32)

src/util/bip152.rs

@@ -20,6 +20,7 @@ use crate::{Block, BlockHash, BlockHeader, Transaction};
 
 /// A BIP-152 error
 #[derive(Clone, PartialEq, Eq, Debug, Copy, PartialOrd, Ord, Hash)]
+#[non_exhaustive]
 pub enum Error {
     /// An unknown version number was used.
     UnknownVersion,
@@ -37,7 +38,16 @@ impl fmt::Display for Error {
 }
 
 #[cfg(feature = "std")]
-impl error::Error for Error {}
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
+impl std::error::Error for Error {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        use self::Error::*;
+
+        match *self {
+            UnknownVersion | InvalidPrefill => None,
+        }
+    }
+}
 
 /// A [PrefilledTransaction] structure is used in [HeaderAndShortIds] to
 /// provide a list of a few transactions explicitly.

src/util/bip158.rs

@@ -62,6 +62,7 @@ const M: u64 = 784931;
 
 /// Errors for blockfilter.
 #[derive(Debug)]
+#[non_exhaustive]
 pub enum Error {
     /// Missing UTXO, cannot calculate script filter.
     UtxoMissing(OutPoint),

src/util/merkleblock.rs

@@ -40,6 +40,8 @@
 //! assert_eq!(1, index[0]);
 //! ```
 
+use core::fmt;
+
 use crate::prelude::*;
 
 use crate::io;
@@ -53,19 +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