Workaround for #10033 (casc_extract: "Invalid file length")

[micolous]

This works around #10033, but doesn't actually fix the bug:

• Makes extract_data() no longer raise an OptionParser parsing error when extraction fails, so the error can be handled better.
• Adds a --continue_on_failure option to casc_extract, which allows batch and unpack operations to continue even if a single file fails to extract.

This also includes a rebased version of #10032, so that it works with current versions of Python.