execErr seems to be ignored other than for exit code

## Expected Behaviour

I would expect us to do _something_ with `execErr` [here](https://github.com/alexellis/go-execute/blob/master/exec.go#L180-L185) unless it can truly be ignored, but its not clear if it can be.

## Current Behaviour

`execErr` is dropped on the floor if its not an exit error.

## Possible Solution

Return it if `ctx.Err()` is nil?

## Steps to Reproduce (for bugs)

N/A

## Context

Just curious as to whether ignoring it is the correct thing to do.

## Your Environment

* `go version` : go version go1.21.3 darwin/arm64

* Operating System and version (e.g. Linux, Windows, MacOS): MacOS Ventura 13.6

* Link to your project or a code example to reproduce issue: N/A