add regression test for rust-bitcoin#806

When parsing a descriptor with `Descriptor::parse_descriptor`, we first parse
as a string and then parse the keys. We fail to consider parsing errors
in the keys, resulting in a panic.

Also, clean up the panic message so it's clearer what's going on.

Author: apoelstra

src/descriptor/mod.rs

@@ -2050,4 +2050,19 @@ pk(03f28773c2d975288bc7d1d205c3748651b075fbc6610e58cddeeddf8f19405aa8))";
         Desc::from_str(&format!("tr({},pk({}))", x_only_key, uncomp_key)).unwrap_err();
         Desc::from_str(&format!("tr({},pk({}))", x_only_key, x_only_key)).unwrap();
     }
+
+    #[test]
+    fn regression_806() {
+        let secp = secp256k1::Secp256k1::signing_only();
+        type Desc = Descriptor<DescriptorPublicKey>;
+        // OK
+        Desc::from_str("pkh(111111111111111111111111111111110000008375319363688624584A111111)")
+            .unwrap_err();
+        // ERR: crashes in translate_pk
+        Desc::parse_descriptor(
+            &secp,
+            "pkh(111111111111111111111111111111110000008375319363688624584A111111)",
+        )
+        .unwrap_err();
+    }
 }

src/lib.rs

@@ -340,10 +340,11 @@ impl<E> TranslateErr<E> {
     ///
     /// This function will panic if the Error is OutError.
     pub fn expect_translator_err(self, msg: &str) -> E {
-        if let Self::TranslatorErr(v) = self {
-            v
-        } else {
-            panic!("{}", msg)
+        match self {
+            Self::TranslatorErr(v) => v,
+            Self::OuterError(ref e) => {
+                panic!("Unexpected Miniscript error when translating: {}\nMessage: {}", e, msg)
+            }
         }
     }
 }