Use addError to set parent exceptions instead of .exception

[DavidTanner]

Currently when an exception is added to a subsegment the Segment.exception doesn't get cleaned up. This causes errors when internal services try to stringify the exception.

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[DavidTanner]

This addresses things like apollographql/apollo-server#1433 on lambda where the Lambda XRay facade doesn't clean up the .exception value that the Subsegment assigns to it.

[willarmiros]

@DavidTanner this appears to be a problem unique to Lambda and the facade segment, is that correct? While I agree that the segment.exception not being cleaned up is a bug, perhaps we should be cleaning it up in the facade segment's reset function which is called at the start of each invocation.

I'm just having a bit of a hard time reasoning about whether this change affects the behavior of addError, namely in this case where segment.exception is already defined and subsegment.addError will point its cause to the ID of the captured exception if the new exception is the same as the existing one.

[DavidTanner]

@willarmiros in that case I think the right thing to do would be to check out the exception is in the segment. I can update the code to check for that.

[NathanielRN]

Hey @DavidTanner ! Are you still interested in working on this PR? If you are please let us know when it's ready for a review 🙂