better parser errors

error_test/parser/struct_errors.sui

@@ -2,7 +2,7 @@
 
 # Missing struct name
 struct
-    x: int
+    x: int,
     y: int
 end
 
@@ -14,17 +14,17 @@ end
 
 # Missing colon in field
 struct Point
-    x int
+    x int,
     y: int
 end
 
 # Missing end keyword
 struct Point
-    x: int
-    y: int
+    x: int,
+    y: int,
 
 # Invalid field separator (semicolon instead of comma)
 struct Point
     x: int;
     y: int
-end
+end