add_group_members_pill: Add button should show error on pending text.

This commit only changes the behaviour for edit screen of this setting.
We'll tackle the create screen in further commits.

web/src/add_group_members_pill.ts

@@ -166,14 +166,23 @@ export function set_up_handlers({
     }
 
     $parent_container.on("keyup", pill_selector, (e) => {
-        if (keydown_util.is_enter_event(e)) {
+        const pill_widget = get_pill_widget();
+        if (!pill_widget.is_pending() && keydown_util.is_enter_event(e)) {
             e.preventDefault();
             callback();
         }
     });
 
     $parent_container.on("click", button_selector, (e) => {
-        e.preventDefault();
+        const pill_widget = get_pill_widget();
-        callback();
+        if (!pill_widget.is_pending()) {
+            e.preventDefault();
+            callback();
+        } else {
+            // We are not appending any value here, but instead this is
+            // a proxy to invoke the error state for a pill widget
+            // that would usually get triggered on pressing enter.
+            pill_widget.appendValue(pill_widget.getCurrentText()!);
+        }
     });
 }