std: Don't use a wrapper for the float error type

Ensures that the same error type is propagated throughout. Unnecessary leakage
of the internals is prevented through the usage of stability attributes.

Closes #24748

src/libstd/error.rs

@@ -147,7 +147,7 @@ impl Error for num::ParseIntError {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Error for num::ParseFloatError {
     fn description(&self) -> &str {
-        self.description()
+        self.__description()
     }
 }