Revert "Response: add set_error()"

This reverts commit 3b0407f.

Author: Fishrock123

src/response.rs

@@ -483,11 +483,6 @@ impl Response {
         self.error.take()
     }
 
-    /// Sets an `Error` on the response, accessible via `error()` and `take_error()`.
-    pub fn set_error(&mut self, error: Error) {
-        self.error = Some(error);
-    }
-
     /// Get the HTTP version, if one has been set.
     ///
     /// # Examples