input: Write a function to classify revenue based on given criteria.
  assert classify_revenue(['Revenue', 0, -1000, 2000, 3000]) == ['Classification', 'Low', 'Loss', 'High', 'High']
  assert classify_revenue([100, 999, 1000, 1999, 2000, 2999, 3000, 4000]) == ['Low', 'Low', 'Medium', 'Medium', 'High', 'High', 'High', 'Error']
  assert classify_revenue([-1, 500, 1500, 2500, 3500]) == ['Loss', 'Low', 'Medium', 'High', 'Error']
output: def classify_revenue(revenues):
      classifications = []
      for revenue in revenues:
          if isinstance(revenue, str):
              classifications.append('Classification')
          elif revenue < 0:
              classifications.append('Loss')
          elif 0 <= revenue < 1000:
              classifications.append('Low')
          elif 1000 <= revenue < 2000:
              classifications.append('Medium')
          elif 2000 <= revenue < 3000:
              classifications.append('High')
          else:
              classifications.append('Error')
      return classifications