refactor: Error message bilingual_str consistency

- Move the decision whether to translate an error message to where it is
  defined. This simplifies call sites: no more `InitError(Untranslated(...))`.

- Make all functions in `util/error.h` consistently return a
  `bilingual_str`. We've decided to use this as error message type so
  let's roll with it.

This has no functional changes: no messages are changed, no new
translation messages are defined.

src/wallet/wallet.cpp

@@ -2997,7 +2997,7 @@ bool CWallet::CreateTransaction(const std::vector<CRecipient>& vecSend, CTransac
     }
 
     if (nFeeRet > m_default_max_tx_fee) {
-        error = Untranslated(TransactionErrorString(TransactionError::MAX_FEE_EXCEEDED));
+        error = TransactionErrorString(TransactionError::MAX_FEE_EXCEEDED);
         return false;
     }