Add exception.cause_class to log entries

What does this MR do and why?

This allows us to more easily find/group errors such as NoMethodError that are re-raised as ActionView::Template::Error

Screenshots or screen recordings

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In Sentry, we do show exception causes:

So we should do the same in Kibana. Currently in Kibana we only have exception.class:

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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