input: Write a class to represent a two-dimensional rectangle with a length and width. The class should include an initializer method to set the length and width. Furthermore, the class should include a method to calculate the area of the rectangle. If the given dimensions are valid (positive integers), return the area, otherwise, return an error message stating that the dimensions are not valid. If either dimension is zero, also return an error message stating that the dimensions cannot be zero.
  class Rectangle:
      def __init__(self, length, width):
          if length <= 0 or width <= 0:
              raise ValueError("The dimensions of the rectangle must be positive and non-zero.")
          self.length = length
          self.width = width
  
      def area(self):
          return self.length * self.width
  
  assert Rectangle(4, 5).area() == 20
  assert Rectangle(6, 3).area() == 18
  try:
      Rectangle(0, 3)
  except ValueError as e:
      assert str(e) == "The dimensions of the rectangle must be positive and non-zero."
  try:
      Rectangle(4, 0)
  except ValueError as e:
      assert str(e) == "The dimensions of the rectangle must be positive and non-zero."
  try:
      Rectangle(-1, 3)
  except ValueError as e:
      assert str(e) == "The dimensions of the rectangle must be positive and non-zero."
output: class Rectangle:
      def __init__(self, length, width):
          if length <= 0 or width <= 0:
              raise ValueError("The dimensions of the rectangle must be positive and non-zero.")
          self.length = length
          self.width = width
  
      def area(self):
          return self.length * self.width