Put a placeholder row into the checkin list to better handle the async checkin option...

Put a placeholder row into the checkin list to better handle the async checkin option (so that items show up in the same order in which they were scanned; it also feels more responsive).  This also forces us into a better way of handling errors: we keep the rows for such scans in the checkin list, but shove the error condition into the alert message field.  These rows can't be acted upon with the Actions for Selected Items menu.

git-svn-id: svn://svn.open-ils.org/ILS/trunk@18534 dcc99617-32d9-48b4-a31d-7c20da2025e4