feat: return detailed error from proctoring provider

[Anas12091101]

Description

Resetting a proctored exam attempt from the new instructor dashboard previously failed with a generic 500 when the proctoring provider was slow/unavailable, leaving the attempt "stuck" with no explanation.

This PR updates the Instructor v2 reset endpoint (SpecialExamResetView in lms/djangoapps/instructor/views/api_v2.py) to catch ProctoredBaseException from remove_exam_attempt and return the exception's own status + message:

except ProctoredBaseException as err:
    return Response({'detail': str(err)}, status=err.http_status)

Now a provider failure returns a 502 with a clear detail (e.g. "The proctoring provider is temporarily unavailable… please try again in a few minutes."), which the instructor-dashboard MFE already displays. Same pattern as the existing ProctoredBaseException handling in this file.

Impacted roles: Instructor/Course Staff (clear error instead of a 500), Operator (failure is now logged/typed).

Supporting information

• Bug: instructor dashboard "reset exam attempt error" (reset 500s, attempt remains) with the Proctortrack provider.
• Depends on openedx/edx-proctoring#1337, which raises the typed BackendProviderCannotRemoveAttempt (502) this view relies on.
• Screenshot

Testing instructions

Automated:

pytest lms/djangoapps/instructor/tests/views/test_special_exams_api_v2.py::SpecialExamResetViewTest

Manual (needs edx-proctoring >= 6.1.0):

1. Create a proctored exam (REST backend) and have a learner start an attempt.
2. Make the provider fail — point the backend base_url at an unreachable host (or stub remove_exam_attempt to raise).
3. As instructor, reset the attempt (dashboard → Special Exams → Exam Attempts).
4. Expect: 502 with {"detail": "…temporarily unavailable…"}, the message shown in the UI, attempt still present, error logged.
5. Regression: with a healthy provider, reset still returns 200 and removes the attempt.

Deadline

None, though it fixes a production-visible bug.

Other information

• Depends on feat: raise descriptive error when proctoring provider cannot remove an attempt edx-proctoring#1337 (safe to merge without it — the except branch just won't trigger).
• No DB migrations. No required config changes.
• Only affects the legacy edx-proctoring reset path, not the edx-exams IDA.

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