use snackbar to show error messages

pdb-js/src/app/app.module.ts

@@ -16,11 +16,13 @@ import {MatButtonModule} from '@angular/material/button';
 import {MatCheckboxModule} from '@angular/material/checkbox';
 import {MatSelectModule} from '@angular/material/select';
 import {MatFormFieldModule, MatInputModule} from '@angular/material';
+import {MatSnackBarModule} from '@angular/material/snack-bar';
 import {MatTooltipModule} from '@angular/material/tooltip';
 import { YAxisRangeComponent } from './y-axis-range/y-axis-range.component';
 import { QueryAutocompleteComponent } from './query-autocomplete/query-autocomplete.component';
 import { LimitByComponent } from './limit-by/limit-by.component';
 import { PlotViewComponent } from './plot-view/plot-view.component';
+import { GalleryViewComponent } from './gallery-view/gallery-view.component';
 
 @NgModule({
   declarations: [
@@ -45,6 +47,7 @@ import { PlotViewComponent } from './plot-view/plot-view.component';
     MatFormFieldModule,
     MatInputModule,
     MatSelectModule,
+    MatSnackBarModule,
     MatTooltipModule,
     BrowserAnimationsModule,
     HttpClientModule