docs: add http guide sample and adjust text (#21326)

PR Close #21326

aio/content/examples/http/src/app/http-error-handler.service.ts

@@ -0,0 +1,47 @@
+import { Injectable } from '@angular/core';
+import { HttpErrorResponse } from '@angular/common/http';
+
+import { Observable } from 'rxjs/Observable';
+import { of } from 'rxjs/observable/of';
+
+import { MessageService } from './message.service';
+
+/** Type of the handleError function returned by HttpErrorHandler.createHandleError */
+export type HandleError =
+  <T> (operation?: string, result?: T) => (error: HttpErrorResponse) => Observable<T>;
+
+/** Handles HttpClient errors */
+@Injectable()
+export class HttpErrorHandler {
+  constructor(private messageService: MessageService) { }
+
+  /** Create curried handleError function that already knows the service name */
+  createHandleError = (serviceName = '') => <T>
+    (operation = 'operation', result = {} as T) => this.handleError(serviceName, operation, result);
+
+  /**
+   * Returns a function that handles Http operation failures.
+   * This error handler lets the app continue to run as if no error occurred.
+   * @param serviceName = name of the data service that attempted the operation
+   * @param operation - name of the operation that failed
+   * @param result - optional value to return as the observable result
+   */
+  handleError<T> (serviceName = '', operation = 'operation', result = {} as T) {
+
+    return (error: HttpErrorResponse): Observable<T> => {
+      // TODO: send the error to remote logging infrastructure
+      console.error(error); // log to console instead
+
+      const message = (error.error instanceof ErrorEvent) ?
+        error.error.message :
+       `server returned code ${error.status} with body "${error.error}"`;
+
+      // TODO: better job of transforming error for user consumption
+      this.messageService.add(`${serviceName}: ${operation} failed: ${message}`);
+
+      // Let the app keep running by returning a safe result.
+      return of( result );
+    };
+
+  }
+}