ProcessGetData(): abort if a block file is missing from disk
LoadBlockIndexDB(): Require block db reindex if any blk*.dat files are missing.
Merge pull request #4173

8c93bf4 LoadBlockIndexDB(): Require block db reindex if any blk*.dat files are missing. (Ashley Holman)
7a0e84d ProcessGetData(): abort if a block file is missing from disk (Ashley Holman)