Don't panic in auth flow (#49916)

Closes #49641

Before you mark this PR as ready for review, make sure that you have:
- [ ] Added a solid test coverage and/or screenshots from doing manual
testing
- [ ] Done a self-review taking into account security and performance
aspects
- [ ] Aligned any UI changes with the [UI
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)

Release Notes:

- Fixed panic when authentication failed to bind to lookback port

crates/client/src/client.rs

@@ -1369,9 +1369,9 @@ impl Client {
                     // zed server to encrypt the user's access token, so that it can'be intercepted by
                     // any other app running on the user's device.
                     let (public_key, private_key) =
-                        rpc::auth::keypair().expect("failed to generate keypair for auth");
+                        rpc::auth::keypair().context("failed to generate keypair for auth")?;
                     let public_key_string = String::try_from(public_key)
-                        .expect("failed to serialize public key for auth");
+                        .context("failed to serialize public key for auth")?;
 
                     if let Some((login, token)) =
                         IMPERSONATE_LOGIN.as_ref().zip(ADMIN_API_TOKEN.as_ref())
@@ -1386,8 +1386,8 @@ impl Client {
                     }
 
                     // Start an HTTP server to receive the redirect from Zed's sign-in page.
-                    let server =
-                        tiny_http::Server::http("127.0.0.1:0").expect("failed to find open port");
+                    let server = tiny_http::Server::http("127.0.0.1:0")
+                        .map_err(|e| anyhow!(e).context("failed to bind callback port"))?;
                     let port = server.server_addr().port();
 
                     // Open the Zed sign-in page in the user's browser, with query parameters that indicate