If [un]archiver tool from custom.ini is not found, try to open same archive using built-in code

[unxed]

For example, command line 7z tool has better .iso support then libarchive. So it is reasonable to add its support to custom.ini as it done in #2220. But if command line 7z is not installed, far2l should try to open .iso using libarchive (or ask user if he wants to install 7z or try to open with libarchive instead). But now it just complains that 7z is not found.