sqlasync: Don't use sqlite3_errstr()

That function was added in 3.7.15 (2012-12-12), so is rather new. It
doesn't really add much, either: It was used as fallback for when
sqlite3_open() gave a NULL object, but that only happens when we're out
of memory. OOM situations aren't handled at all yet, and even in that
case a manual static string would work just as fine as sqlite3_errstr().