Update Lesson 2 Task 4 to work without libraries

Lesson_2/task4.py

@@ -2,24 +2,19 @@
 # Author: Stanislav Mykhailenko
 # License: Unlicense
 
-# Return codes:
-# 0 - OK
-# 1 - Invalid number entered
-
-import sys
-
-def numberError():
-	print("Invalid number entered.")
-	sys.exit(1)
+error = False
 
 try: number = int(input("Enter a natural number: "))
-except ValueError: numberError()
+except ValueError: error = True
 
-if number < 1: numberError()
+if not error and number < 1: error = True
 
+if not error:
 	factorial = 1
 
 	for i in range(2, number + 1):
 		factorial = factorial * i
 	else:
 		print(str(number) + " factorial is " + str(factorial))
+else:
+	print("Invalid number entered.")