Autoprune

This pull extends #4515 and #4481 implementing a lightweight sanity check which executes periodically during runtime to make sure the block files needed by the node are present.

There's also a new mode of operation introduced: -pruned, which automatically removes old block files and marks the blocks as pruned.

This should allow to lower the amount of resources needed to run a node.

See the individual commits, and the other pulls for details about all the changes introduced.
I'd just say "Error checking required block files" in both cases. I wouldn't suggest using -pruned as a way to get around errors caused by random disk corruption (for example). Someone that wants to run a pruned node will make a conscious choice to do so.