Switch away from exceptions in refactored tx code

After refactoring general-purpose PSBT and transaction code out of RPC code,
for use in the GUI, it's no longer appropriate to throw exceptions. Instead we
now return bools for success, and take an output parameter for an error object.
We still use JSONRPCError() for the error objects, since only RPC callers
actually care about the error codes.

src/wallet/rpcwallet.cpp

@@ -13,6 +13,7 @@
 #include <validation.h>
 #include <key_io.h>
 #include <net.h>
+#include <node/transaction.h>
 #include <outputtype.h>
 #include <policy/feerate.h>
 #include <policy/fees.h>
@@ -4003,7 +4004,11 @@ UniValue walletprocesspsbt(const JSONRPCRequest& request)
     // Fill transaction with our data and also sign
     bool sign = request.params[1].isNull() ? true : request.params[1].get_bool();
     bool bip32derivs = request.params[3].isNull() ? false : request.params[3].get_bool();
-    bool complete = FillPSBT(pwallet, psbtx, nHashType, sign, bip32derivs);
+    bool complete = true;
+    TransactionError err;
+    if (!FillPSBT(pwallet, psbtx, err, complete, nHashType, sign, bip32derivs)) {
+        throw JSONRPCTransactionError(err);
+    }
 
     UniValue result(UniValue::VOBJ);
     CDataStream ssTx(SER_NETWORK, PROTOCOL_VERSION);
@@ -4117,7 +4122,11 @@ UniValue walletcreatefundedpsbt(const JSONRPCRequest& request)
 
     // Fill transaction with out data but don't sign
     bool bip32derivs = request.params[4].isNull() ? false : request.params[4].get_bool();
-    FillPSBT(pwallet, psbtx, 1, false, bip32derivs);
+    bool complete = true;
+    TransactionError err;
+    if (!FillPSBT(pwallet, psbtx, err, complete, 1, false, bip32derivs)) {
+        throw JSONRPCTransactionError(err);
+    }
 
     // Serialize the PSBT
     CDataStream ssTx(SER_NETWORK, PROTOCOL_VERSION);