tree-wide: make use of log_error_errno() return value

Turns this:

        r = -errno;
        log_error_errno(errno, "foo");

into this:

        r = log_error_errno(errno, "foo");

and this:

        r = log_error_errno(errno, "foo");
        return r;

into this:

        return log_error_errno(errno, "foo");