[Test] verify that diagnostic events cannot fail

[jayz22]

Diagnostic events are non-metered, not part of the protocol. So if they fail, they will trigger the error path, which will cause validators different behaviors depending on whether diagnostics are enabled or not.
We should test/audit all the diagnostic event generating paths to make sure they are infallible.
cc @sisuresh

[sisuresh]

yeah this is something @dmkozh also mentioned to me recently. We should just swallow errors instead of propagating them.

[jayz22]

Resolved by #1123 and #1138