feat: throw on non-zero exit

[43081j]

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:

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
  }
}

cc @benmccann

[benmccann]

Looks good to me! It'd probably be good to add some docs

[43081j]

have updated to be a throwOnError option rather than a breaking change