Add sanity-check exception

Found by mypy while adding types.

jedi/api/__init__.py

@@ -105,6 +105,9 @@ class Script:
         self.path = path.absolute() if path else None
 
         if code is None:
+            if path is None:
+                raise ValueError("Must provide at least one of code or path")
+
             # TODO add a better warning than the traceback!
             with open(path, 'rb') as f:
                 code = f.read()