tests: improve error handling in ExampleRunner

[jcsp]

It's unclear why, but in a case where this threw from _worker, the test ended up hanging during teardown.

We don't need to throw from _worker: it is neater to capture the exception and promptly raise it in the test body via condition_met.

Fixes https://github.com/redpanda-data/redpanda/issues/7426

• [ ] none - not a bug fix
• [ ] none - issue does not exist in previous branches
• [ ] none - papercut/not impactful enough to backport
• [x] v22.3.x
• [x] v22.2.x
• [ ] v22.1.x

UX Changes

None

Release Notes

• none

[jcsp]

CI failure is https://github.com/redpanda-data/redpanda/issues/7043

[jcsp]

/backport v22.2.x

[jcsp]

/backport v22.3.x