show error message when dashboard cannot be loaded

pdb-js/src/app/dashboard-page/dashboard/dashboard.component.html

@@ -4,20 +4,32 @@
     height: 100%;
     
   }
-  .spinner {
+  .center {
     position: absolute;
     top: 50%;
     left: 50%;
     transform: translate(-50%, -50%);
   }
+  .center-content {
+    text-align: center;
+  }
+  .is-error {
+    font-size: 3rem;
+    color: #333;
+  }
 
   .content {
     padding: 0.5em;
   }
 </style>
-<div *ngIf="dashboard === undefined" class="spinner">
+
+<div *ngIf="dashboard === undefined && !error" class="center">
   <mat-spinner></mat-spinner>
 </div>
+<div *ngIf="error" class="center center-content">
+  <div class="is-error">{{error}}</div>
+  <div>Try another <a [routerLink]="['/dashboard']">dashboard</a>.</div>
+</div>
 <div *ngIf="dashboard !== undefined" class="content">
   <div class="toolbar">
     <button mat-button  (click)="addText()">Add Text</button>