Add MatDialog and use messageService to display errors

src/app/services/message.service.ts

@@ -1,11 +1,17 @@
 import { Injectable } from '@angular/core';
+import { Subject } from 'rxjs';
 
-@Injectable()
+@Injectable({
+  providedIn: 'root'
+})
 export class MessageService {
   messages: string[] = [];
+  private messageAddedSource = new Subject<string>();
+  messageAdded$ = this.messageAddedSource.asObservable();
 
   add(message: string) {
     this.messages.push(message);
+    this.messageAddedSource.next(message);
   }
 
   clear() {