Fix a swallowed error and properly raise StandardError exceptions

What does this MR do and why?

This missing line in tests was causing the tests to fail since client.issue_notes/client.issue_discussions would return nil, which doesn't reply to #auto_paginate. In this case the test were wrong (not the implementation).

I also made GitlabClient#handle_gitlab_client_exceptions raise StandardError after it rescues these errors, otherwise it could swallow legit bugs.

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