Zimit2: add more resiliency / automatic detection of missing ZIM entries

[benoit74]

See https://github.com/openzim/warc2zim/pull/218#issuecomment-2020609892 for details.

Basically we probably need to:

• merge the indexed_urls and existing_zim_paths into a single dictionary zim_entries_created where the key is the ZIM path and the value a boolean indicating if the entry has been added to the ZIM (or is only "expected")
• raise a fatal error when at the end of the conversion we still have entries that have not yet been added to the ZIM (this is not normal, so we will have a ZIM with broken links)