Fix error handling on keyword blocks

Some blocks (menu, layout, etc) would emit an error if they didn't find
an opening brace, which would fail if, for example, you had an object
with ID "menu" and tried to use it as a property value.

gtkblueprinttool/extensions/gtk_a11y.py

@@ -164,7 +164,7 @@ a11y = Group(
     A11y,
     Sequence(
         Keyword("accessibility", True),
-        OpenBlock().expected("`{`"),
+        OpenBlock(),
         Until(a11y_prop, CloseBlock()),
     )
 )

gtkblueprinttool/extensions/gtk_layout.py

@@ -62,7 +62,7 @@ layout = Group(
     Layout,
     Sequence(
         Keyword("layout", True),
-        OpenBlock().expected("`{`"),
+        OpenBlock(),
         Until(layout_prop, CloseBlock()),
     )
 )

gtkblueprinttool/extensions/gtk_menu.py

@@ -112,7 +112,7 @@ menu_item_shorthand = Group(
 )
 
 menu_contents.children = [
-    OpenBlock().expected("`{`"),
+    OpenBlock(),
     Until(AnyOf(
         menu_section,
         menu_submenu,