Add few tests checking error codes

[lampajr]

Changes proposed

• [x] Add couple of tests checking the error code
• [x] Fetch the openapi only if not already present in the local filesystem

Check List (Check all the applicable boxes)

• [x] My code follows the code style of this project.
• [ ] My change requires changes to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] All new and existing tests passed.