Clarify code in HTTPUClient.Do.

The previous code was misleading by returning the `err` variable rather than an explicit `nil`. This code path was actually the success path. Functionally it was correct as that specific scoping of `err` could only be `nil` if that line of code was reached.

httpu/httpu.go

@@ -128,5 +128,7 @@ func (httpu *HTTPUClient) Do(req *http.Request, timeout time.Duration, numSends
 
 		responses = append(responses, response)
 	}
-	return responses, err
+
+	// Timeout reached - return discovered responses.
+	return responses, nil
 }