Ignore known error codes from the rostering services

[marcospri]

We'll tackle this individually as we gather more information are able to reproduce and fix them.

See https://hypothes-is.slack.com/archives/C1MA4E9B9/p1725372178973579

[sentry-io[bot]]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: lms/services/roster.py

Function	Unhandled Issue
fetch_assignment_roster	ExternalRequestError: ExternalRequestError(message=None, request=Request(method='GET', url='https://hypothesis.instruct... ... Event Count: 4
fetch_assignment_roster	ExternalRequestError: ExternalRequestError(message=None, request=Request(method='GET', url='https://canvas.umn.edu/api/... ... Event Count: 1

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