input: Write a python function to download a webpage and save its content to a file.
  assert download_webpage("https://example.com", "example_website.html") == "File downloaded successfully."
  assert download_webpage("https://www.google.com", "google.html") == "File downloaded successfully."
  assert download_webpage("https://www.wikipedia.org", "wikipedia.html") == "File downloaded successfully."
output: def download_webpage(url, filename):
      import requests
      
      try:
          response = requests.get(url)
          response.raise_for_status()
          with open(filename, 'w', encoding='utf-8') as file:
              file.write(response.text)
          return "File downloaded successfully."
      except Exception as e:
          return f"An error occurred: {e}"