[Obs AI Assistant] Show error toast on failure (#179335)

If the call to `ObservabilityAIAssistantAppService.start` fails, show an
error toast to notify the user.

<img width="354" alt="Screenshot 2024-03-25 at 12 23 46"
src="https://github.com/elastic/kibana/assets/2564140/021d442c-ec34-4acf-b441-6005acde1666">

x-pack/plugins/observability_solution/observability_ai_assistant_app/public/components/nav_control/index.tsx

@@ -10,6 +10,7 @@ import { EuiButton } from '@elastic/eui';
 import { css } from '@emotion/react';
 import { v4 } from 'uuid';
 import useObservable from 'react-use/lib/useObservable';
+import { i18n } from '@kbn/i18n';
 import { useObservabilityAIAssistantAppService } from '../../hooks/use_observability_ai_assistant_app_service';
 import { ChatFlyout } from '../chat/chat_flyout';
 import { useKibana } from '../../hooks/use_kibana';
@@ -22,6 +23,7 @@ export function NavControl({}: {}) {
 
   const {
     services: {
+      notifications,
       plugins: {
         start: {
           observabilityAIAssistant: { ObservabilityAIAssistantChatServiceContext },
@@ -36,7 +38,23 @@ export function NavControl({}: {}) {
 
   const chatService = useAbortableAsync(
     ({ signal }) => {
-      return hasBeenOpened ? service.start({ signal }) : undefined;
+      return hasBeenOpened
+        ? service.start({ signal }).catch((error) => {
+            notifications.toasts.addError(error, {
+              title: i18n.translate(
+                'xpack.observabilityAiAssistant.navControl.initFailureErrorTitle',
+                {
+                  defaultMessage: 'Failed to initialize Observability AI Assistant',
+                }
+              ),
+            });
+
+            setHasBeenOpened(false);
+            setIsOpen(false);
+
+            throw error;
+          })
+        : undefined;
     },
     [service, hasBeenOpened]
   );