input/tidal: add exception class TidalError

Allows catchers to inspect the HTTP status.

src/input/plugins/TidalErrorParser.hxx

@@ -30,7 +30,8 @@ template<typename T> struct ConstBuffer;
 struct StringView;
 
 /**
- * Parse an error JSON response.
+ * Parse an error JSON response and throw a #TidalError upon
+ * completion.
  */
 class TidalErrorParser final : public YajlResponseParser {
 	const unsigned status;