Merge rust-bitcoin/rust-bitcoin#1049: Implement std::error::Error for ParseAmount

5ce3401 Implement std::error::Error for ParseAmount (Tobin C. Harding)

Pull request description:

  The `ParseAmountError` does not implement `std::error::Error`, must have been missed when we did the rest.

  Implement `std::error::Error` for `ParseAmount`.

ACKs for top commit:
  Kixunil:
    ACK 5ce3401
  apoelstra:
    ACK 5ce3401

Tree-SHA512: 8dafc472b7c23b54d856344e786e0f22e8e179f30f6c1011fbf5f8f0c6b1b5d74ed8e4f2638e5f8246f04dbb429e60027db6fe584d51a78957a6e904feb9e8a3

Author: apoelstra

src/util/amount.rs

@@ -185,7 +185,22 @@ impl fmt::Display for ParseAmountError {
 
 #[cfg(feature = "std")]
 #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
-impl std::error::Error for ParseAmountError {}
+impl std::error::Error for ParseAmountError {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        use self::ParseAmountError::*;
+
+        match *self {
+            Negative
+                | TooBig
+                | TooPrecise
+                | InvalidFormat
+                | InputTooLarge
+                | InvalidCharacter(_)
+                | UnknownDenomination(_)
+                | PossiblyConfusingDenomination(_) => None
+        }
+    }
+}
 
 fn is_too_precise(s: &str, precision: usize) -> bool {
     s.contains('.') || precision >= s.len() || s.chars().rev().take(precision).any(|d| d != '0')