More detailed serial config error

Marlin/src/HAL/TEENSY35_36/HAL.h

@@ -72,6 +72,8 @@ extern USBSerialType USBSerial;
 #elif WITHIN(SERIAL_PORT, 0, 3)
   #define MYSERIAL1 MSERIAL(SERIAL_PORT)
   DECLARE_SERIAL(SERIAL_PORT);
+#else
+  #error "SERIAL_PORT must be from 0 to 3, or -1 for Native USB."
 #endif
 
 #define HAL_SERVO_LIB libServo