x.json2: improve error message upon missing comma (#20602)

vlib/x/json2/decoder.v

@@ -407,10 +407,10 @@ fn (mut p Parser) decode_object() !Any {
 		p.next_with_err()!
 		// step 3 -> value
 		fields[cur_key] = p.decode_value()!
-		if p.tok.kind != .comma && p.tok.kind != .rcbr {
-			return UnknownTokenError{
+		if p.tok.kind !in [.comma, .rcbr] {
+			return InvalidTokenError{
 				token: p.tok
-				kind: .object
+				expected: .comma
 			}
 		} else if p.tok.kind == .comma {
 			p.next_with_err()!

vlib/x/json2/tests/decoder_test.v

@@ -82,3 +82,29 @@ fn test_raw_decode_array_invalid() {
 	}
 	assert false
 }
+
+struct ContactItem {
+	description string
+	telnr       string
+}
+
+struct User {
+	name    string
+	age     int
+	contact ContactItem
+}
+
+fn test_decode_missing_comma() {
+	data := '{ 
+				"name": "Frodo", 
+				"age": 25 
+				"contact": {
+					"description": "descr",
+					"telnr": "+32333"
+				}
+			}'
+	user := json.decode[User](data) or {
+		assert err.msg().contains('invalid token')
+		return
+	}
+}