Add Error Handling to Python Cipher Scripts

Description:
Enhance the reliability of the existing Python cipher scripts (e.g., Caesar, AES) by adding input validation and exception handling. This improves the user experience and script robustness.

Expected Tasks:

• Review all Python files in the ciphers/ folder
• Add try-except blocks to handle:
  • Non-string input
  • Incorrect key sizes
  • Unsupported characters
• Write meaningful error messages and avoid crashes
• Create a new file ERROR_HANDLING.md to document:
  • Types of handled errors
  • Example error messages

Stretch Tasks:

• Use custom exception classes (e.g., InvalidKeyError)
• Add unit tests to verify error handling works