input: Write a function to create a web crawler in Python that starts from a given URL, searches the page for other links, and then scrapes the page content. If the given URL is not provided, return a message indicating that the URL is required. If the URL is valid, return the scraped content from all visited pages.
  assert crawl("https://example.com")=={"https://example.com": "content of example.com", "https://example.com/page1": "content of example.com/page1"}
  assert crawl("")=="URL is required."
  assert crawl("https://invalidurl.com")=="Please provide a valid URL."
output: def crawl(url):
      if not url:
          return "URL is required."
      try:
          import requests
          from bs4 import BeautifulSoup
          from urllib.parse import urljoin
          visited = {}
          to_visit = [url]
          
          while to_visit:
              current_url = to_visit.pop(0)
              if current_url in visited:
                  continue
              response = requests.get(current_url)
              if response.status_code != 200:
                  return "Please provide a valid URL."
              soup = BeautifulSoup(response.text, 'html.parser')
              visited[current_url] = soup.get_text()
              for link in soup.find_all('a', href=True):
                  new_url = urljoin(current_url, link['href'])
                  if new_url not in visited:
                      to_visit.append(new_url)
          return visited
      except Exception:
          return "Please provide a valid URL."