add error instance to unhandled error perf metrics

[EdmondChuiHW]

Summary

Adding a new field to the error evens to capture the Error instance. We can investigate stacktraces from it.

This is possible because Errors are serialisable across postMessage/iframe. Pretty cool!

Test plan

Test E2E via D58141961

Upstreaming plan

• This commit should be sent as a patch to the upstream devtools-frontend repo. I've reviewed the contribution guide.
• This commit is React Native-specific and cannot be upstreamed.