Add Error Handling, Input Validation, and Offline Mode with Caching

[Stonebanks-js]

Description:

We need to enhance the functionality of the currency conversion script by:

• Implementing proper error handling for invalid inputs, network failures, or API errors.
• Adding input validation to ensure correct currency codes and numerical values are provided.
• Extending the cache mechanism to allow for offline mode by storing conversion rates locally.

Feature Details:

Error Handling:

• Catch exceptions related to invalid currency codes or issues with the API.

• Handle scenarios where the API returns no data or fails to connect.

Input Validation:

• Ensure currency codes are three-letter strings (ISO format).
• Validate that the amount entered is a positive integer or float.

Offline Mode with Cache:

• Store the exchange rates in a local file after the first API call.
• Use the cached file for future conversions when offline or if the API fails.

Steps that I will perform :

• [ ] 1. > Modify the Currency.py and Currency_calculator.py files.
• [ ] 2. > Implement error handling in the conversion methods.
• [ ] 3. > Add a mechanism to save and retrieve rates from a local file in the cache system.

[Stonebanks-js]

@DhanushNehru Assign this task to me and add @hacktoberfest-accepted label