tools: bpftool: add error message on pin failure

No error message is currently printed if the pin syscall
itself fails. It got lost in the loadall refactoring.

Fixes: 77380998

 ("bpftool: add loadall command")
Reported-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>