Fix reporter commenting on wrong discussions

This fixes the regex that attempts to match existing discussions with new errors. It now works as expected with multiline strings.

It also now displays a warning if the expected error is not found in an existing discussion.

Here's a test showing:

• an invalid comment before the fix (the error didn't match so it should have been a new discussion): https://gitlab.com/mlapierre-test/testcases-test/-/issues/216#note_315976243
• a new discussion after the fix: https://gitlab.com/mlapierre-test/testcases-test/-/issues/216#note_315980382

Closes: https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/455