fix importfeed --force skip behavior reversion

fix importfeed --force skip behavior reversion

importfeed --force: Don't treat it as a failure when an already downloaded
file exists. (Fixes a behavior change introduced in 10.20230626.)

04ee6c4c6b2 caused the reversion. Inside a CommandPerform, stop causes it
to fail. Before that commit, it was inside a CommandStart, where stop
causes it to skip.