[grouping] Buggy edge cases in filter_exceptions_for_exception_groups - no/empty exception 0

[lobsterkatie]

TL;DR: In our chained-exception strategy, we're not handling the case of one or more outer wrapper errors being gone very well.

Relates to this KeyError event, which is erroring out here because there is no entry 0 in exception_tree (which is a dict). Values are added to exception_tree a little higher up in the function, and they're based on mechanism.exception_id and mechanism.parent_id.

I went and checked out the issue that was being processed, and indeed, neither of the exceptions would put anything at 0:

Seems like we peeled off more than one outer/wrapper exception.

Not high priority, because it's only happened once that we know of, but something we might want to make more robust regardless.

UPDATE: The same project (but a different event, from a different SDK) caused a related error a few lines later. The JSON was equivalent to the above, except one of the parent_ids was 0 (though there was no actual exception 0 in the data). As a result, exception_tree[0] was an empty ExceptionTreeNode (from here), which then made the if block creating top_level_exceptions not run, which then made iterating through it a few lines later error out.