Response: add set_error()

This has been has somewhat controversial in the development of
#174
but was ultimately deemed as desirable due to the idea of giving
preference to an existing Response rather than creating a new one
in upstream libraries.

Author: Fishrock123

src/response.rs

@@ -483,6 +483,11 @@ 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