wallet: Avoid translating RPC errors when creating txs

Also, mark feebumper bilingual_str as Untranslated

They are technical and have previously not been translated either.
It is questionable whether they can even appear in the GUI.

src/wallet/test/wallet_tests.cpp

@@ -516,7 +516,7 @@ public:
         CTransactionRef tx;
         CAmount fee;
         int changePos = -1;
-        std::string error;
+        bilingual_str error;
         CCoinControl dummy;
         {
             BOOST_CHECK(wallet->CreateTransaction({recipient}, tx, fee, changePos, error, dummy));