Continue after external unpacker failed

[StatusQuo]

After marking several archives and extracting them (Alt+F9) using an external unpacker (like WinRAR), several instances of the external unpacker are called one after another.
If one of the unpack operations quits with an error (like missing write access to the target), unpacking of the other marked archives is skipped, too (TC 7.50).

I suggest to ask, if the user wants to
- Skip the failed extraction (continue with the next), or
- Cancel the whole unpacking task.
- Retry option would be nice, too.