input: Write a python function to check if a URL is active and responding.
  assert check_url("http://www.example.com")=="URL is active and responding"
  assert check_url("http://www.invalidurl.com").startswith("Error:")
output: def check_url(url):
      import requests
      try:
          response = requests.get(url)
          if response.status_code == 200:
              return "URL is active and responding"
          else:
              return f"Error: Received status code {response.status_code}"
      except requests.exceptions.RequestException as e:
          return f"Error: {e}"