fix: #226 Support default response status

[ad-walker]

Issue/feature request: https://github.com/openapi-library/OpenAPIValidators/issues/226

• default response status when provided will catch unspecified response codes
• Updated tests and error messages
• Incremented port number of exampleApp to 5001 to avoid UPNP conflicts
• Changes to issues template applied by linter

cc: @rwalle61 based on comments re: original issue

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (9511b66) to head (3c88cbd).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #271 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 17 17 Lines 418 419 +1 Branches 63 65 +2 ========================================= + Hits 418 419 +1 ```

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