Use explicit error::Error impl instead of the default

In a further effort to make the code brain-dead easy to read; use an
explicit implementation of `std::error::Error` that returns `None`
instead of relying on the default trait implementation.

bitcoin/src/bip152.rs

@@ -329,7 +329,9 @@ impl fmt::Display for TxIndexOutOfRangeError {
 }
 
 #[cfg(feature = "std")]
-impl error::Error for TxIndexOutOfRangeError {}
+impl error::Error for TxIndexOutOfRangeError {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> { None }
+}
 
 /// A [BlockTransactions] structure is used to provide some of the transactions
 /// in a block, as requested.

bitcoin/src/blockdata/locktime/absolute.rs

@@ -637,7 +637,9 @@ impl fmt::Display for ConversionError {
 }
 
 #[cfg(feature = "std")]
-impl std::error::Error for ConversionError {}
+impl std::error::Error for ConversionError {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> { None }
+}
 
 /// Describes the two types of locking, lock-by-blockheight and lock-by-blocktime.
 #[derive(Debug, Clone, Copy, Eq, PartialEq, Hash)]
@@ -679,7 +681,9 @@ impl fmt::Display for OperationError {
 }
 
 #[cfg(feature = "std")]
-impl std::error::Error for OperationError {}
+impl std::error::Error for OperationError {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> { None }
+}
 
 #[cfg(test)]
 mod tests {

bitcoin/src/pow.rs

@@ -717,7 +717,9 @@ impl fmt::Display for TryFromError {
 }
 
 #[cfg(feature = "std")]
-impl std::error::Error for TryFromError {}
+impl std::error::Error for TryFromError {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> { None }
+}
 
 impl Add for U256 {
     type Output = Self;

bitcoin/src/psbt/mod.rs

@@ -890,7 +890,9 @@ impl fmt::Display for ExtractTxError {
 }
 
 #[cfg(feature = "std")]
-impl std::error::Error for ExtractTxError {}
+impl std::error::Error for ExtractTxError {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> { None }
+}
 
 /// Input index out of bounds (actual index, maximum index allowed).
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -931,7 +933,9 @@ impl fmt::Display for IndexOutOfBoundsError {
 }
 
 #[cfg(feature = "std")]
-impl std::error::Error for IndexOutOfBoundsError {}
+impl std::error::Error for IndexOutOfBoundsError {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> { None }
+}
 
 #[cfg(feature = "base64")]
 mod display_from_str {