DEV: Improve logging of errors in `Jobs::ProcessUserNotificationSchedules` (PR #13802)

Gives us the actual error and backtrace to work with. Otherwise, the logging of the error is not useful at all.

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@martin-brennan That is super fast :slight_smile: Thank you for the review :+1:

@tgxworld no problem thanks for telling me about the much more useful Discourse.warn_exception haha