Improve formatting of error messages (error names)

[qmonnet]

Following the discussion with Andrii on the BPF mailing list, we want to improve error messages by writing explicitly the name of the error returned from the kernel in addition to its interpretation, for example:

Error: can't get next program: [-EPERM] Operation not permitted

There's some work to do in libbpf (teaching libbpf_strerror_r() to do this), and then we could wrap error messages in bpftool to use that function.