validation: report if pruning prevents completion of verification

Now the verifychain RPC returns false if the checks didn't
finish because the blocks requested to be queried have been pruned.

src/validation.h

@@ -354,6 +354,7 @@ enum class VerifyDBResult {
     CORRUPTED_BLOCK_DB,
     INTERRUPTED,
     SKIPPED_L3_CHECKS,
+    SKIPPED_MISSING_BLOCKS,
 };
 
 /** RAII wrapper for VerifyDB: Verify consistency of the block and coin databases */