Show the real problem instead of just saying "No such file or directory".

Now will print below info:
libbpf: failed to find '.BTF' ELF section in /home/changbin/work/linux/vmlinux
Error: failed to load BTF from /home/changbin/work/linux/vmlinux: No such file or directory
Signed-off-by: NChangbin Du <changbin.du@gmail.com>
Signed-off-by: NAndrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20221217223509.88254-2-changbin.du@gmail.com