scripted diff: Improve invalid vout value rpc error message

-BEGIN VERIFY SCRIPT-
r() { sed -i 's/vout must be positive/vout cannot be negative/g' $1 }
r $(git grep -l 'vout must be positive')
-END VERIFY SCRIPT-

src/bitcoin-tx.cpp

@@ -594,7 +594,7 @@ static void MutateTxSign(CMutableTransaction& tx, const std::string& flagStr)
 
             const int nOut = prevOut["vout"].get_int();
             if (nOut < 0)
-                throw std::runtime_error("vout must be positive");
+                throw std::runtime_error("vout cannot be negative");
 
             COutPoint out(txid, nOut);
             std::vector<unsigned char> pkData(ParseHexUV(prevOut["scriptPubKey"], "scriptPubKey"));