Add TransactionGasPriceError to LibExchangeRichErrors

contracts/exchange-libs/contracts/src/LibExchangeRichErrors.sol

@@ -131,6 +131,10 @@ library LibExchangeRichErrors {
     // bytes4(keccak256("TransactionExecutionError(bytes32,bytes)"))
     bytes4 internal constant TRANSACTION_EXECUTION_ERROR_SELECTOR =
         0x20d11f61;
+    
+    // bytes4(keccak256("TransactionGasPriceError(bytes32,unt256,uint256)"))
+    bytes4 internal constant TRANSACTION_GAS_PRICE_ERROR_SELECTOR =
+        0xa26dac09;
 
     // bytes4(keccak256("IncompleteFillError(uint8,uint256,uint256)"))
     bytes4 internal constant INCOMPLETE_FILL_ERROR_SELECTOR =
@@ -293,6 +297,14 @@ library LibExchangeRichErrors {
         return BATCH_MATCH_ORDERS_ERROR_SELECTOR;
     }
 
+    function TransactionGasPriceErrorSelector()
+        internal
+        pure
+        returns (bytes4)
+    {
+        return TRANSACTION_GAS_PRICE_ERROR_SELECTOR;
+    }
+
     function BatchMatchOrdersError(
         BatchMatchOrdersErrorCodes errorCode
     )
@@ -581,6 +593,23 @@ library LibExchangeRichErrors {
         );
     }
 
+    function TransactionGasPriceError(
+        bytes32 transactionHash,
+        uint256 actualGasPrice,
+        uint256 requiredGasPrice
+    )
+        internal
+        pure
+        returns (bytes memory)
+    {
+        return abi.encodeWithSelector(
+            TRANSACTION_GAS_PRICE_ERROR_SELECTOR,
+            transactionHash,
+            actualGasPrice,
+            requiredGasPrice
+        );
+    }
+
     function IncompleteFillError(
         IncompleteFillErrorCode errorCode,
         uint256 expectedAssetFillAmount,