RTU: Check CRC before signalling an exception response.

The first five response ADU bytes received are checked for a valid
function code, assuming an error if it is unknown.  The received ADU
part is then treated as an exception frame.  But even those must pass
the CRC validation, otherwise the supposed "wrong" function code
cannot even be trusted at all.

This change adds the CRC check for the first five bytes, falling
through to further processing when it fails.  The overall CRC and
function code is checked later in parse_response_adu(), so an invalid
code will be detected again at that point.

This is mainly relevant if the responses are received partially from
the serial input buffer, which will cause many CRC and parsing
failures.  In those cases, the reported exception will likely be a
CRCError or ValueError for the whole ADU, instead of an invalid
function code based on only the first 5 bytes.

umodbus/client/serial/rtu.py

@@ -45,6 +45,7 @@
 import struct
 
 from umodbus.client.serial.redundancy_check import get_crc, validate_crc
+from umodbus.client.serial.redundancy_check import CRCError
 from umodbus.functions import (create_function_from_response_pdu,
                                expected_response_pdu_size_from_request_pdu,
                                pdu_to_function_code_or_raise_error, ReadCoils,
@@ -199,7 +200,13 @@ def raise_for_exception_adu(resp_adu):
     :raises ModbusError: When a response contains an error code.
     """
     resp_pdu = resp_adu[1:-2]
-    pdu_to_function_code_or_raise_error(resp_pdu)
+    try:
+        validate_crc(resp_adu)
+        pdu_to_function_code_or_raise_error(resp_pdu)
+    except CRCError:
+        # The response cannot be trusted at all, so ignore any
+        # possibly invalid function code.
+        pass
 
 
 def send_message(adu, serial_port):