Make past spork indexer handle bootstrapping like live indexer

[Ullaakut]

Goal of this PR

Fixes #499

Makes the past spork indexer behave like the live spork indexer when it comes to bootstrapping:

• If an index already exists but we can't read from it, fail
• If the index is empty and no checkpoint is given, fail
• If the index is empty, load the given checkpoint
• If the index is not empty and that a checkpoint is given, use the checkpoint to initialize the loader and then load from the index
• If the index is not empty and that no checkpoint is given, start from the index

Doing things this way makes the --force option not needed anymore, since we can now combine having an index and a checkpoint at the same time.

Checklist

• [x] Is on the right branch
• [x] Documentation is up-to-date
• [x] Tests are up-to-date