Revert stabilization of feature(never_type).

This commit is just covering the feature gate itself and the tests
that made direct use of `!` and thus need to opt back into the
feature.

A follow on commit brings back the other change that motivates the
revert: Namely, going back to the old rules for falling back to `()`.

src/libstd/error.rs

@@ -233,7 +233,7 @@ impl<'a> From<Cow<'a, str>> for Box<Error> {
     }
 }
 
-#[stable(feature = "never_type", since = "1.26.0")]
+#[unstable(feature = "never_type", issue = "35121")]
 impl Error for ! {
     fn description(&self) -> &str { *self }
 }