some error messages in APIs not in json

[aanchal22]

Describe the problem
REST API returns some HTML error messages that should be json.

To Reproduce
Steps to reproduce the behavior:

1. PUT /shopcarts/<int:customer_id>/<int:product_id>
2. Make an erroneous request
3. See error (not in JSON)

Expected behavior
The error message should be in JSON format.

Observed behavior
Error message not in JSON format.

Additional context
We need to include error_handlers.py in init.py and also check code coverage after testing our API error responses.