linux: Show warning if file picker portal is missing (#14401)

This PR adds a warning when the file chooser couldn't be opened on Linux

It's quite confusing when trying to open a file and apparently nothing
happens:

fixes https://github.com/zed-industries/zed/issues/11089,
https://github.com/zed-industries/zed/issues/14328,
https://github.com/zed-industries/zed/issues/13753#issuecomment-2225812703,
https://github.com/zed-industries/zed/issues/13766,
https://github.com/zed-industries/zed/issues/14384,
https://github.com/zed-industries/zed/issues/14353,
https://github.com/zed-industries/zed/issues/9209


![image](https://github.com/user-attachments/assets/5acabdaa-7a9d-4225-9480-e371d20387c3)


Release Notes:

- N/A

crates/gpui/src/platform/test/platform.rs

@@ -34,7 +34,7 @@ pub(crate) struct TestPlatform {
 #[derive(Default)]
 pub(crate) struct TestPrompts {
     multiple_choice: VecDeque<oneshot::Sender<usize>>,
-    new_path: VecDeque<(PathBuf, oneshot::Sender<Option<PathBuf>>)>,
+    new_path: VecDeque<(PathBuf, oneshot::Sender<Result<Option<PathBuf>>>)>,
 }
 
 impl TestPlatform {
@@ -80,7 +80,7 @@ impl TestPlatform {
             .new_path
             .pop_front()
             .expect("no pending new path prompt");
-        tx.send(select_path(&path)).ok();
+        tx.send(Ok(select_path(&path))).ok();
     }
 
     pub(crate) fn simulate_prompt_answer(&self, response_ix: usize) {
@@ -216,14 +216,14 @@ impl Platform for TestPlatform {
     fn prompt_for_paths(
         &self,
         _options: crate::PathPromptOptions,
-    ) -> oneshot::Receiver<Option<Vec<std::path::PathBuf>>> {
+    ) -> oneshot::Receiver<Result<Option<Vec<std::path::PathBuf>>>> {
         unimplemented!()
     }
 
     fn prompt_for_new_path(
         &self,
         directory: &std::path::Path,
-    ) -> oneshot::Receiver<Option<std::path::PathBuf>> {
+    ) -> oneshot::Receiver<Result<Option<std::path::PathBuf>>> {
         let (tx, rx) = oneshot::channel();
         self.prompts
             .borrow_mut()