Add capability to send a custom default response

[shubhendumadhukar]

Is your feature request related to a problem? Please describe.
In cases Camouflage can not find a mockfile for an invoked endpoint, it sends following default response

{
    "error": "Not Found"
}

User should, however, be able to override this default response with custom default responses at global level and at the API level.

Describe the solution you'd like
Implementation as follows:

1. User invokes a GET call to /default/error/response
2. Corresponding mock file is evaluated as ${MOCK_DIR}/default/error/response/GET.mock
3. Camouflage can not find the mock file, however it finds a directory ${MOCK_DIR}/default
4. Camouflage determines ${MOCK_DIR}/default/__/GET.mock as the closest match for the invoked API.
5. Camouflage looks for an API level default response in the closest match.
6. If above mock file too, is unavailable, Camouflage looks for a global default response ${MOCK_DIR}/__/GET.mock
7. Finally, if all determined mock files are unavailable, Camouflage sends its inbuilt default response as shown above.

Describe alternatives you've considered
NA

Additional context
NA