log: Set errno when qb_log_target_alloc() fails

The callers of qb_log_target_alloc() return -errno when it fails.
However, qb_log_target_alloc() wasn't setting errno.

The only failure case is when QB_TARGET_LOG_MAX (32) logs have been opened, so
it's unlikely to ever be a real-world problem. But in that case, now set errno
to EMFILE ("Too many open files").