Adds custom error message for expects

What does this MR do and why?

The custom error message is to help with not creating duplicate failure issues (for example, Failure in browser_ui/8_monitor/incident_manage... (#408118 - closed) and Failure in browser_ui/8_monitor/incident_manage... (#407810 - closed)) Since the incident name is generated by Faker, the error message differs every time which makes the bot think it's a new failure.

MR acceptance checklist

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