No error is shown when trying to save an invalid language code for a Lemma on a Lexeme. We should show an error to give the editor some indication about what's going on.
**Steps to reproduce:**
* Go to https://test.wikidata.org/wiki/Lexeme:L1 and start editing the lemma
* Enter "test" as the spelling variant
* Press enter or click publish
Expected behaviour: The language code is not valid, so an error should be shown.
Actual behaviour: It briefly looks like it's saving and then it returns to the previous state with no indication of anything having gone wrong.
GIVEN a Lemma on a Lexeme
WHEN saving an invalid language code
THEN an error is shown
AND the edit is not saved
AND edit mode is restored
[ ] error message is shown when trying to save an invalid language code for a Lemma on a Lexeme