Improve coverage special_numbers (TheAlgorithms#12414)

* Improve coverage bell_numbers

* improve more function

* Update hamming_numbers.py

---------

Co-authored-by: Maxim Smolskiy <mithridatus@mail.ru>

maths/special_numbers/bell_numbers.py

@@ -21,6 +21,10 @@ def bell_numbers(max_set_length: int) -> list[int]:
         list: A list of Bell numbers for sets of lengths from 0 to max_set_length.
 
     Examples:
+    >>> bell_numbers(-2)
+    Traceback (most recent call last):
+        ...
+    ValueError: max_set_length must be non-negative
     >>> bell_numbers(0)
     [1]
     >>> bell_numbers(1)

maths/special_numbers/hamming_numbers.py

@@ -13,6 +13,10 @@ def hamming(n_element: int) -> list:
     :param n_element: The number of elements on the list
     :return: The nth element of the list
 
+    >>> hamming(-5)
+    Traceback (most recent call last):
+        ...
+    ValueError: n_element should be a positive number
     >>> hamming(5)
     [1, 2, 3, 4, 5]
     >>> hamming(10)
@@ -22,7 +26,7 @@ def hamming(n_element: int) -> list:
     """
     n_element = int(n_element)
     if n_element < 1:
-        my_error = ValueError("a should be a positive number")
+        my_error = ValueError("n_element should be a positive number")
         raise my_error
 
     hamming_list = [1]

maths/special_numbers/harshad_numbers.py

@@ -11,6 +11,8 @@ def int_to_base(number: int, base: int) -> str:
     Where 'base' ranges from 2 to 36.
 
     Examples:
+    >>> int_to_base(0, 21)
+    '0'
     >>> int_to_base(23, 2)
     '10111'
     >>> int_to_base(58, 5)
@@ -26,6 +28,10 @@ def int_to_base(number: int, base: int) -> str:
     Traceback (most recent call last):
         ...
     ValueError: 'base' must be between 2 and 36 inclusive
+    >>> int_to_base(-99, 16)
+    Traceback (most recent call last):
+        ...
+    ValueError: number must be a positive integer
     """
 
     if base < 2 or base > 36:
@@ -101,6 +107,8 @@ def harshad_numbers_in_base(limit: int, base: int) -> list[str]:
     Traceback (most recent call last):
         ...
     ValueError: 'base' must be between 2 and 36 inclusive
+    >>> harshad_numbers_in_base(-12, 6)
+    []
     """
 
     if base < 2 or base > 36: