input: Write a python function to read the contents of a file and print them, while handling the case where the file might not exist.
  assert read_file("existing_file.txt") == "This is an existing file."
  assert read_file("non_existing_file.txt") == "Error: file not found!"
  assert read_file("") == "Error: file not found!"
output: def read_file(filename):
      try:
          with open(filename, 'r') as file:
              return file.read()
      except FileNotFoundError:
          return "Error: file not found!"