[wallet] abort when attempting to fund a transaction above maxtxfee

FundTransaction calls GetMinimumFee which, when the fee rate is absurdly high, quietly reduced the fee to -maxtxfee. Becaue an absurdly high fee rate is usually the result of a fat finger, aborting seems safer behavior.

src/qt/walletmodel.cpp

@@ -221,9 +221,7 @@ WalletModel::SendCoinsReturn WalletModel::prepareTransaction(WalletModelTransact
             return TransactionCreationFailed;
         }
 
-        // reject absurdly high fee. (This can never happen because the
-        // wallet caps the fee at m_default_max_tx_fee. This merely serves as a
-        // belt-and-suspenders check)
+        // Reject absurdly high fee
         if (nFeeRequired > m_wallet->getDefaultMaxTxFee())
             return AbsurdFee;
     }