Report an error when trying to open ui in linux::headless (#11952)

Release Notes:

- N/A

crates/gpui/src/platform.rs

@@ -108,6 +108,9 @@ pub(crate) trait Platform: 'static {
     fn displays(&self) -> Vec<Rc<dyn PlatformDisplay>>;
     fn primary_display(&self) -> Option<Rc<dyn PlatformDisplay>>;
     fn active_window(&self) -> Option<AnyWindowHandle>;
+    fn can_open_windows(&self) -> anyhow::Result<()> {
+        Ok(())
+    }
     fn open_window(
         &self,
         handle: AnyWindowHandle,