typecheck: diagnose unknown return types

tools/validate_typecheck_diagnostics.py

@@ -173,6 +173,9 @@ EXPECTED = {
     "tests/typecheck_invalid_expected_type_bad.claro": [
         "tests/typecheck_invalid_expected_type_bad.claro:2: CHECK TYPE needs a known type such as NUMBER, TEXT, YESNO, LIST, or MAP, but BANANA is not a Claro type.",
     ],
+    "tests/typecheck_invalid_return_type_bad.claro": [
+        "tests/typecheck_invalid_return_type_bad.claro:1: Function square declares an unknown return type BANANA. Use a Claro type such as NUMBER, TEXT, YESNO, LIST, or MAP.",
+    ],
     "tests/typecheck_object_field_unknown_object_bad.claro": [
         "tests/typecheck_object_field_unknown_object_bad.claro:5: Object player is not known yet. Create it with NEW ClassName player before setting player.score.",
     ],