add exercise for exception handling

Author: tnahddisttud

Basics/Exercise/15_exception_handling/exception_handing_exercise.md

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+## Exercise: Python Exception Handling
+
+1. Write a Python program that takes a numeric grade from the user (between 0 and 100), and prints the corresponding letter grade:
+
+```
+90–100 → A  
+80–89  → B  
+70–79  → C  
+60–69  → D  
+<60    → F
+```
+
+2. Your program should handle the following exceptions:
+   - If the user enters a non-numeric value, catch the `ValueError` and display a user-friendly message.
+   - If the user enters a number outside the valid range (0 to 100), raise a `ValueError` yourself with a custom message.
+
+3. Use the `try–except–else–finally` structure:
+   - `try`: Attempt to parse the input and compute the letter grade.
+   - `except`: Handle conversion errors and invalid ranges.
+   - `else`: Print the final grade if everything was successful.
+   - `finally`: Print a goodbye message like `"Thank you for using the Grade Calculator. Goodbye!"` no matter what.
+
+[Solution](https://gist.github.com/your-solution-link-here)

Basics/Exercise/15_exception_handling/exception_handling_solution.py

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+def calculate_letter_grade(score: int) -> str:
+    if 90 <= score <= 100:
+        return "A"
+    elif 80 <= score <= 89:
+        return "B"
+    elif 70 <= score <= 79:
+        return "C"
+    elif 60 <= score <= 69:
+        return "D"
+    else:
+        return "F"
+
+try:
+    user_input = input("Enter your grade (0–100): ")
+    grade = int(user_input)
+
+    if grade < 0 or grade > 100:
+        raise ValueError("Grade must be between 0 and 100.")
+
+    letter = calculate_letter_grade(grade)
+except ValueError as ve:
+    print(f"⚠️ {ve}")
+else:
+    print(f"✅ Your letter grade is: {letter}")
+finally:
+    print("🎓 Thank you for using the Grade Calculator. Goodbye!")