generator: explicitly fail if ROM is detected to be on system root

- Until now, this wasn't captured.
- It wasn't a big deal on Windows, but on everything else this caused
  the '/' character to be completely removed from the ROM path, which
  generated something similar to "sd:ROMsNDSMario Kart DS.nds".
- Make the program exit when it is detected not to be on the SD card,
  or if a custom path is not set.

generator/generator.py

@@ -271,6 +271,9 @@ def getrompath(self, path) -> str:
                     break
                 temp = direc
             root = temp
+        if root == "C:" or root == "/":
+            self.message("This ROM is not on your SD card!\nPlease use a ROM on your SD card, or set a custom path.")
+            exit()
         path = unicodedata.normalize("NFC", os.path.abspath(path))
         path = path.replace(root, "")
         if os.name == 'nt':