input: Write a function to create a web scraper that extracts specific data from a given website using the BeautifulSoup package in Python. The function should parse the webpage, find the required HTML elements using tags and attributes, and then store the extracted data in the desired format. If the input website URL is not provided or is invalid, the function should return an error message.
  
  assert web_scraper("<url_of_webpage>")==["<url1>", "<url2>", "<url3>"]
  assert web_scraper("")=="Error: URL not provided"
  assert web_scraper("invalid_url")=="Error: Invalid URL"
output: def web_scraper(url):
      if not url:
          return "Error: URL not provided"
      try:
          import requests
          from bs4 import BeautifulSoup
          response = requests.get(url)
          response.raise_for_status()
          soup = BeautifulSoup(response.text, 'html.parser')
          links = soup.find_all('a', href=True)
          return [link['href'] for link in links]
      except:
          return "Error: Invalid URL"