libbpf: Stringify errno in log messages in the remaining code

Convert numeric error codes into the string representations in log
messages in the rest of libbpf source files.

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20241111212919.368971-5-mykyta.yatsenko5@gmail.com