Simplify return for error::Error impl for string::ParseError

Author: dirk

src/libstd/error.rs

@@ -185,7 +185,7 @@ impl Error for string::FromUtf16Error {
 #[stable(feature = "str_parse_error2", since = "1.8.0")]
 impl Error for string::ParseError {
     fn description(&self) -> &str {
-        "parse error"
+        match *self {}
     }
 }