More detailed serial config error

Marlin/src/HAL/TEENSY31_32/HAL.cpp

@@ -35,6 +35,8 @@
 #define IMPLEMENT_SERIAL(X)  _IMPLEMENT_SERIAL(X)
 #if WITHIN(SERIAL_PORT, 0, 3)
   IMPLEMENT_SERIAL(SERIAL_PORT);
+#else
+  #error "SERIAL_PORT must be from 0 to 3."
 #endif
 USBSerialType USBSerial(false, SerialUSB);