Introduces a new `throwOnError` option which will cause tinyexec to
throw any time a non-zero exit code is encountered.

If the exit code is `null`, we will not throw since it means something
went very wrong anyway (the process is still running and shouldn't be,
since we saw the `close` event by then).

If the exit code is greater than `0`, we will throw a
`NonZeroExitError` which has an `exitCode` property.

For example:

```ts
try {
  await x('foo', [], {throwOnError: true});
} catch (err) {
  if (err instanceof NonZeroExitCode) {
    err.exitCode; // the exit code
    err.result; // the tinyexec process
    err.result.killed; // getters on tinyexec process
  }
}
```