Improve diagnostic header UI (#9888)

This PR rearranges the diagnostics to put the headers to the left of the
diagnostic messages and adds an additional button to close the
diagnostics.

<img width="394" alt="Screenshot 2024-03-27 at 2 01 19 PM"
src="https://github.com/zed-industries/zed/assets/2280405/83be4051-6441-47c6-9b48-77c75ce9c8eb">

<img width="326" alt="Screenshot 2024-03-27 at 2 01 56 PM"
src="https://github.com/zed-industries/zed/assets/2280405/d849ca34-91e9-4de6-9d9c-503b75e97d60">

As a drive by, I also quieted a useless but loud log message.

Release Notes:

- Added a close button to the `f8` diagnostics.

crates/copilot/src/copilot.rs

@@ -797,7 +797,7 @@ impl Copilot {
     ) -> Task<Result<()>> {
         let server = match self.server.as_authenticated() {
             Ok(server) => server,
-            Err(error) => return Task::ready(Err(error)),
+            Err(_) => return Task::ready(Ok(())),
         };
         let request =
             server