Avoid dropping 'Error' inside '#[launch]' future.

Due to tokio-rs/tokio#4780, a panicking top-level future combined with
an uncooperative background task prevents runtime shutdown. To avoid
this in the case of `Rocket::launch()` returning an `Error`, which
panics on drop if it isn't inspected, we return the `Result` to the
caller (i.e., `main`) instead of the `block_on` future. This prevent the
panic from occuring inside of the `block_on` future and so the runtime
terminates even with uncooperative I/O.

core/codegen/src/attribute/entry/launch.rs

@@ -42,6 +42,11 @@ impl EntryAttr for Launch {
             ___rocket
         });
 
+        let launch = match f.sig.asyncness {
+            Some(_) => quote_spanned!(ty.span() => async move { #rocket.launch().await }),
+            None => quote_spanned!(ty.span() => #rocket.launch()),
+        };
+
         let (vis, mut sig) = (&f.vis, f.sig.clone());
         sig.ident = syn::Ident::new("main", sig.ident.span());
         sig.output = syn::ReturnType::Default;
@@ -51,7 +56,7 @@ impl EntryAttr for Launch {
             #[allow(dead_code)] #f
 
             #vis #sig {
-                ::rocket::async_main(async move { let _res = #rocket.launch().await; })
+                let _ = ::rocket::async_main(#launch);
             }
         ))
     }

core/lib/src/server.rs

@@ -529,13 +529,13 @@ impl Rocket<Orbit> {
                                 Ok(rocket)
                             }
                             Err(rocket) => {
-                                warn!("Server failed to shutdown cooperatively.");
+                                warn!("Shutdown failed: outstanding background I/O.");
                                 Err(Error::shutdown(rocket, None))
                             }
                         }
                     }
                     _ = &mut shutdown_timer => {
-                        warn!("Server failed to shutdown cooperatively.");
+                        warn!("Shutdown failed: server executing after timeouts.");
                         return Err(Error::shutdown(rocket.clone(), None));
                     },
                 }