Bug fixes and dependency update

• context.DeadlineExceeded is now preserved across the network, including keeping its reference so it can be compared by reference (#89).
• a bug was fixed whereby a call cycle (and stack overflow) would happen when calling errors.Is on an error that delegates its Error method to Format or SafeFormatError (#90).
• the errors library does not any more depend on a custom fork of sentry-go, and now uses the upstream library directly (#92). Special thanks to @jhchabran for this contribution.