Only implement Error if std feature is enabled.

This should rightfully have been caught by the CI in the original PR,
but alas it was not. My bad for not seeing it manually.

See rust-lang/rust#53166 for issue on this
kind of mistake not causing an error like it used to.

src/lib.rs

@@ -588,6 +588,7 @@ where
     }
 }
 
+#[cfg(feature = "std")]
 impl<E> std::error::Error for Error<E>
 where
     E: std::error::Error