validation: return VerifyDBResult::INTERRUPTED if verification was interrupted

This means that the -verifydb RPC will now return false if it
cannot finish due to the node being shutdown.

src/validation.h

@@ -352,6 +352,7 @@ arith_uint256 CalculateHeadersWork(const std::vector<CBlockHeader>& headers);
 enum class VerifyDBResult {
     SUCCESS,
     CORRUPTED_BLOCK_DB,
+    INTERRUPTED,
 };
 
 /** RAII wrapper for VerifyDB: Verify consistency of the block and coin databases */