-import { Observable } from 'rxjs'
-import { catchError, map, switchMap } from 'rxjs/operators'
-import { HttpClient, HttpParams, HttpRequest } from '@angular/common/http'
+import { Observable, of, throwError } from 'rxjs'
+import { catchError, map, mergeMap, switchMap } from 'rxjs/operators'
+import { HttpClient, HttpErrorResponse, HttpParams, HttpRequest } from '@angular/common/http'