NPA-6425: auditing error responses (#319)

# Pull Request

## 🧾 Ticket Link

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https://nhsd-jira.digital.nhs.uk/browse/NPA-6425

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## 📄 Description/Summary of Changes

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purpose/scope/impact of the changes -->

- This PR matches up the error responses we return from VRS with the
Spec.
- This involved some error codes changing, and some errors being
added/removed
- A PR has been raised in the PVRS repo to reflect some of these changes
too.

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## 🧪 Developer Testing Carried Out

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the ticket. Include: -->
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- Tested this spec branch against my PVRS changes to check all tests
passing
NHSDigital/proxy-validated-relationships-service#1392

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Author: ehallam

.gitignore

@@ -16,6 +16,7 @@ __pycache__/
 .venv/
 
 smoketest-report.xml
+test-report.xml
 env
 .dir-locals.el
 *.pyc