tools/bpftool: Fix error handing in do_skeleton()

Fix pass 0 to PTR_ERR, also dump more err info using
libbpf_strerror.

Fixes: 5dc7a8b2

 ("bpftool, selftests/bpf: Embed object file inside skeleton")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20200717123059.29624-1-yuehaibing@huawei.com