Propagate service error when gracefully shutting down

# Motivation

Propagate the service error when we shutdown the group after the service threw.

Author: FranzBusch

Sources/ServiceLifecycle/ServiceGroup.swift

@@ -338,6 +338,7 @@ public actor ServiceGroup: Sendable {
                                 group: &group,
                                 gracefulShutdownManagers: gracefulShutdownManagers
                             )
+                            return .failure(error)
                         } catch {
                             return .failure(error)
                         }

Tests/ServiceLifecycleTests/ServiceGroupTests.swift

@@ -433,7 +433,7 @@ final class ServiceGroupTests: XCTestCase {
             ]
         )
 
-        await withThrowingTaskGroup(of: Void.self) { group in
+        try await withThrowingTaskGroup(of: Void.self) { group in
             group.addTask {
                 try await serviceGroup.run()
             }
@@ -474,6 +474,10 @@ final class ServiceGroupTests: XCTestCase {
 
             // Let's exit from the first service
             await service1.resumeRunContinuation(with: .success(()))
+
+            try await XCTAsyncAssertThrowsError(await group.next()) {
+                XCTAssertTrue($0 is ExampleError)
+            }
         }
     }