Write/read multiple registers doesn't return exception code 0x03

[stephane]

When the number of write registers is set to MODBUS_MAX_WR_WRITE_REGISTERS + 1 the error is catched by modbus_receive and the exception code isn't returned by modbus_response so illegal data exception code isn't returned (see comment in unit tests).

[stephane]

Same problem for modbus_read_bits and modbus_read_registers