fix: reject unsupported SKK expressions

tests/data/skk/source.skk.utf8

@@ -3,4 +3,4 @@
 えがお /笑顔;face/
 かんじ /感じ/漢字;duplicate/
 きごう /記号;symbol/普通/
-むこう /候補 with space/(concat "式" "候補")/[無効]/#0/
+むこう /候補 with space/

tests/skk2ktrans_test.py

@@ -36,6 +36,13 @@ class Skk2KtransTest(unittest.TestCase):
 		self.assertNotEqual(result.returncode, 0)
 		self.assertIn(b"escaped candidates are unsupported", result.stderr)
 
+	def test_expression_candidate_is_rejected(self):
+		source = b'key /(concat "a/b")/literal/\n'
+		result = subprocess.run(
+			[CONVERTER], input=source, capture_output=True, check=False)
+		self.assertNotEqual(result.returncode, 0)
+		self.assertIn(b"expression candidates are unsupported", result.stderr)
+
 
 if __name__ == "__main__":
 	unittest.main()