Expose exception_code to API. (#2615)

Author: janiversen

pymodbus/pdu/pdu.py

@@ -35,6 +35,7 @@ def __init__(self,
         self.registers: list[int] = registers or []
         self.count: int = count or len(self.registers)
         self.status: int = status
+        self.exception_code: int = 0
         self.fut: asyncio.Future
 
     def isError(self) -> bool: