input: Write a function to scrape an online news website to get the headlines. The function should take a URL as input and return a list of all the headlines found on that page. Use the `requests` library to fetch the webpage content and the `BeautifulSoup` library to parse the content and extract the headlines.
  assert scrape_headlines("https://www.example-news.com/") != []
  assert isinstance(scrape_headlines("https://www.example-news.com/")[0], str)
  assert scrape_headlines("https://www.nonexistent-url.com/") == []
output: import requests
  from bs4 import BeautifulSoup
  
  def scrape_headlines(url):
      try:
          response = requests.get(url)
          response.raise_for_status()
          soup = BeautifulSoup(response.content, 'html.parser')
          headlines = [headline.text.strip() for headline in soup.find_all('h1') + soup.find_all('h2') + soup.find_all('h3')]
          return headlines
      except:
          return []