Fix spy.toString when getters on a spy's thisValue throw

[flotwig]

Purpose (TL;DR)

Fix #2215 by ignoring errors thrown from property getters - since the algo is just looking for thisValue[prop] === this, errors are irrelevant.

Background (Problem in detail)

This was uncovered when trying to spy on the Notification constructor in #2215, but this could happen with any object. The tests for this PR include such an example.

Solution

Ignore the error. There is nothing to do.

How to verify

1. Check out this branch
2. npm install
3. run the test without the changes, then run the test with the changes

Checklist for author

• npm run lint passes
• References to standard library functions are cached.

[mroderick]

Thank you for taking the time to give both an in-depth error report and a great pull request 💯

[mroderick]

This has been published as part of sinon@9.0.0