tools, bpftool: Print correct error message when failing to load BTF

btf__parse_raw and btf__parse_elf return negative error numbers wrapped
in an ERR_PTR, so the extracted value needs to be negated before passing
them to strerror which expects a positive error number.

Before:
  Error: failed to load BTF from .../vmlinux: Unknown error -2

After:
  Error: failed to load BTF from .../vmlinux: No such file or directory

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200525135421.4154-1-tklauser@distanz.ch
Signed-off-by: Alexei Starovoitov <ast@kernel.org>