Enabling blockfilterindex after pruning has begun prevents startup

[luke-jr]

https://x.com/privacymatter21/status/1823784670905163824

"basic block filter index best block of the index goes beyond pruned data. Please disable the index or reindex (which will download the whole blockchain again)"

This prevents starting the GUI entirely.

Possible solutions:

• Disable the peerblockfilters checkbox if any blocks are pruned.
• Prompt to reindex or disable peerblockfilters (and blockfilterindex) at startup.
• Setup prune locks and re-download as needed.