Just tested for practice, the script stops, provides warning, notifies user of non-existant packages in the list, allows user to proceed or exit. That's decent, user can then exit and modify the list file.
nanomeister
junk
Warning: The above files in /tmp/nanotiny.lst do not exist in dCore repos.
To proceed anyway press Enter, (n)o exits:
tc@box:/tmp$ cat nanotiny.lst
nano-tiny
nanomeister
junk
This was with a new list, not sure about re-importing. jls were you speaking of a new list or performing an update/re-import?