Merge #15408: Remove unused TransactionError constants

fa9b60c842 Remove unused TransactionError constants (MarcoFalke)

Pull request description:

  Fixup to #14978, which introduced a bunch of unused enum values, such as `UNKNOWN_ERROR`, `ERROR_COUNT` and `TRANSACTION_ERR_LAST`. None of those have a meaning in the context of an `enum class`, where the compiler can infer if all cases have been covered in a switch-case.

  Also, move the global `::maxTxFee` back to the rpc caller, so it can be set on a per call basis (in the future).

Tree-SHA512: 7f1e2d795f1c1278ecd54ddab2b92c2a862f3c637b482d1d008208925befa1c9dd4b3c4bb1bfcbc5ca4b66a41004aaf01ea96ea95236f944250b8a6cf99ff173

src/psbt.cpp

@@ -309,21 +309,19 @@ bool FinalizeAndExtractPSBT(PartiallySignedTransaction& psbtx, CMutableTransacti
     return true;
 }
 
-bool CombinePSBTs(PartiallySignedTransaction& out, TransactionError& error, const std::vector<PartiallySignedTransaction>& psbtxs)
+TransactionError CombinePSBTs(PartiallySignedTransaction& out, const std::vector<PartiallySignedTransaction>& psbtxs)
 {
     out = psbtxs[0]; // Copy the first one
 
     // Merge
     for (auto it = std::next(psbtxs.begin()); it != psbtxs.end(); ++it) {
         if (!out.Merge(*it)) {
-            error = TransactionError::PSBT_MISMATCH;
-            return false;
+            return TransactionError::PSBT_MISMATCH;
         }
     }
     if (!out.IsSane()) {
-        error = TransactionError::INVALID_PSBT;
-        return false;
+        return TransactionError::INVALID_PSBT;
     }
 
-    return true;
+    return TransactionError::OK;
 }