Improve the rollover script for waiting for the final deposits
to be ingested into `l2geth` by ensuring that it handles the case
that not all deposits have been ingested in a safe way. Previously
the error message was ominous, which leads to confusion when running
the script. Now instead of erroring, reset the loop and attempt to
look for the final deposit again. Also add a bunch of comments
to make the code easier to understand.

This script is used as part of the upgrade to bedrock.