Added sad paths to handle and excetion for merchantitems and itemmerhants

[MDelarosa1993]

This code defines a test for the Api::V1::MerchantItemsController to ensure that it handles exceptions gracefully when a non-existent item is requested. It verifies that:

• HTTP Status: The response status code is 404 when the item is not found.
• Error Response Structure: The JSON response contains a message key with a string value and an errors key that is an array.
• Error Details: Each error object in the errors array contains status and title keys, both with string values.

[jimmacur]

Thanks for the update! I see that the redundant Item.find call was included by mistake. Removing the extra find call should clean up the code. The new tests for error handling look good and will help ensure robustness.