Return error message when performance bar group is not found

What does this MR do?

Two commits: the first one returns a proper error when the performance bar settings in the admin area are invalid. (Either the group doesn't exist, or the group is set but the box isn't checked.)

The second is a convenience as people have been adding a trailing slash, which was failing. We can always safely remove a trailing slash and look up the group without it.

For #33448 (closed).

Screenshots

Type group name, don't enable performance bar:

Tick box, invalid group:

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.