Have testproposedblock return RPC_VERIFY_ALREADY_IN_CHAIN

Instead of RPC_VERIFY_ERROR when the block is already in the chain.

src/rpc/mining.cpp

@@ -1439,7 +1439,7 @@ UniValue testproposedblock(const JSONRPCRequest& request)
     uint256 hash = block.GetHash();
     BlockMap::iterator mi = mapBlockIndex.find(hash);
     if (mi != mapBlockIndex.end())
-        throw JSONRPCError(RPC_VERIFY_ERROR, "already have block");
+        throw JSONRPCError(RPC_VERIFY_ALREADY_IN_CHAIN, "already have block");
 
     CBlockIndex* const pindexPrev = chainActive.Tip();
     // TestBlockValidity only supports blocks built on the current Tip