descriptor: fix key parsing error handling in parse_desc

Author: apoelstra

src/descriptor/mod.rs

@@ -771,7 +771,7 @@ impl Descriptor<DescriptorPublicKey> {
         let descriptor = Descriptor::<String>::from_str(s)?;
         let descriptor = descriptor
             .translate_pk(&mut keymap_pk)
-            .map_err(|e| e.expect_translator_err("No Outer context errors"))?;
+            .map_err(TranslateErr::flatten)?;
 
         Ok((descriptor, keymap_pk.0))
     }

src/lib.rs

@@ -365,6 +365,17 @@ impl<E> TranslateErr<E> {
     }
 }
 
+impl TranslateErr<Error> {
+    /// If we are doing a translation where our "outer error" is the generic
+    /// Miniscript error, eliminate the `TranslateErr` type which is just noise.
+    pub fn flatten(self) -> Error {
+        match self {
+            Self::TranslatorErr(e) => e,
+            Self::OuterError(e) => e,
+        }
+    }
+}
+
 impl<E> From<E> for TranslateErr<E> {
     fn from(v: E) -> Self { Self::TranslatorErr(v) }
 }