Raise a runtime error when converting missing character

I'm not 100% sure this is correct, but I think a None value would
lead to an error later on anyways.

Author: maresb

barcode/codex.py

@@ -211,9 +211,9 @@ def _convert(self, char):
                     value = int(self._buffer)
                     self._buffer = ""
                     return value
-                return None
-            return None
-        return None
+        raise RuntimeError(
+            f"Character {char} could not be converted in charset {self._charset}."
+        )
 
     def _try_to_optimize(self, encoded):
         if encoded[1] in code128.TO: