input: Write a python function to fetch and save the contents of a given URL to an HTML file every 5 minutes.
  assert fetch_and_save("https://example.com", "output.html") == "File saved successfully"
  assert fetch_and_save("https://nonexistentwebsite.com", "output.html") == "Error fetching webpage."
output: def fetch_and_save(url, filename):
      import requests
      import time
  
      while True:
          try:
              response = requests.get(url)
              response.raise_for_status()
              with open(filename, 'w', encoding='utf-8') as file:
                  file.write(response.text)
              return "File saved successfully"
          except requests.RequestException:
              return "Error fetching webpage"
          time.sleep(300)