Update to use net.ErrClosed

[dlm]

In #50 the user @dimalinux proposed updating from an error string the the net.ErrClosed library. As mentioned by @dimalinux, this change keeps the error string the same, while updating the behavior of the vsock library to be similar to the standard library. One benefit is that with this change, a user can now check for the error via

errors.Is(err, net.ErrClosed)

instead of comparing error strings.

I only made the change in the code but I am happy to add a test for this but I was not sure where the correct place would be. Any guidance on that would be great!