coro-spawn does not handle errors from uv.spawn

[squeek502]

This line:

local handle, pid = uv.spawn(path, options, onExit)

can error and will return nil, errmsg (e.g. ENOENT: no such file or directory). Currently, coro-spawn will just continue with that and the coroutine will never resume. Would submit a PR fix, but not sure how the fix should be implemented.