Log outpoint for failed Sapling witness lookup

src/wallet/asyncrpcoperation_sendmany.cpp

@@ -435,7 +435,9 @@ bool AsyncRPCOperation_sendmany::main_impl() {
         // Add Sapling spends
         for (size_t i = 0; i < notes.size(); i++) {
             if (!witnesses[i]) {
-                throw JSONRPCError(RPC_WALLET_ERROR, "Missing witness for Sapling note");
+                throw JSONRPCError(RPC_WALLET_ERROR,
+                    strprintf( "Missing witness for Sapling note at outpoint %s", notes[i].op.ToString())
+                );
             }
             assert(builder_.AddSaplingSpend(expsk, notes[i], anchor, witnesses[i].get()));
         }