files-view: Don't add non-existent files

Under certain circumstances (i.e. moving a file), we end up adding a non-existent file. This causes selection problems because the non-existent file is selected and then removed (causing another file to be selected). I'm not sure where the root cause is (between directory-async, directory notify, etc), but lets simply not add a file that we know shouldn't be added.

Fixes: #2828 (closed)