Drop the Sized constraint on impl Error for Box<T>

Because of the implicit `Sized` constraint, `Box<Error>` does not
implement `Error`, which is potentially surprising when trying to pass
to a type which requires `T: Error`.

Author: sgrif

src/libstd/error.rs

@@ -278,7 +278,7 @@ impl Error for char::DecodeUtf16Error {
 }
 
 #[stable(feature = "box_error", since = "1.7.0")]
-impl<T: Error> Error for Box<T> {
+impl<T: Error + ?Sized> Error for Box<T> {
     fn description(&self) -> &str {
         Error::description(&**self)
     }