Panic when system is out of resources (unwrap)

https://github.com/getsentry/sentry-rust/blob/96e1da3d4d5d4197899cab8cfdcfcdb672689e65/sentry-core/src/session.rs#L228

We're seeing processes fail with this message:
```
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Os {
code: 11, kind: WouldBlock, message: "Resource temporarily unavailable" }',

/sentry-core-0.27.0/src/session.rs:228:14
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

Unfortunately I do not have a full backtrace at this time.