feat(arr): add Error::Decode for bad subtitle bytes

crates/arr-subs/src/error.rs

@@ -74,6 +74,19 @@ pub enum Error {
         #[source]
         source: std::io::Error,
     },
+
+    /// A downloaded subtitle's bytes did not decode under any plausible
+    /// encoding.
+    ///
+    /// Writing mojibake into the library is worse than a failed attempt
+    /// (#186 already records a failure reason).
+    #[error("{candidate} did not decode to UTF-8: {detail}")]
+    Decode {
+        /// The candidate whose bytes failed to decode.
+        candidate: CandidateId,
+        /// What was wrong, short enough to log.
+        detail: String,
+    },
 }
 
 impl Error {