We don't really end need error after removing the assertion so we can get rid of it.

I suggest adding in sterror(errno) to the warning, the extra information will come in handy.

My reading of ERR(3) suggests this is what warn(...) does, but perhaps I'm misunderstanding.

The err(), errc(), verr(), verrc(), warn(), warnc(), vwarn(), and vwarnc() functions append an error message obtained from strerror(3) based on a supplied error code value or the global variable errno, preceded by another colon and space unless the fmt argument is NULL.