Auto merge of #3592 - Eirik0:z-sendmany-better-error, r=bitcartel

Better error message when sending to both sprout and sapling

When trying to send to both Sprout and Sapling (not currently supported with z_sendmany) we were getting the following error in our operation result: `general exception: boost::bad_get: failed value get using boost::get`.

This PR changes this to fail with a better error message before the async operation begins:
```
error code: -8
error message:
Cannot send to both Sprout and Sapling addresses using z_sendmany
```

src/wallet/rpcwallet.cpp

@@ -3687,6 +3687,9 @@ UniValue z_sendmany(const UniValue& params, bool fHelp)
     std::vector<SendManyRecipient> zaddrRecipients;
     CAmount nTotalOut = 0;
 
+    bool containsSproutOutput = false;
+    bool containsSaplingOutput = false;
+
     for (const UniValue& o : outputs.getValues()) {
         if (!o.isObject())
             throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, expected object");
@@ -3710,6 +3713,16 @@ UniValue z_sendmany(const UniValue& params, bool fHelp)
                 bool toSprout = !toSapling;
                 noSproutAddrs = noSproutAddrs && toSapling;
 
+                containsSproutOutput |= toSprout;
+                containsSaplingOutput |= toSapling;
+
+                // Sending to both Sprout and Sapling is currently unsupported using z_sendmany
+                if (containsSproutOutput && containsSaplingOutput) {
+                    throw JSONRPCError(
+                        RPC_INVALID_PARAMETER,
+                        "Cannot send to both Sprout and Sapling addresses using z_sendmany");
+                }
+
                 // If we are sending from a shielded address, all recipient
                 // shielded addresses must be of the same type.
                 if (fromSprout && toSapling) {