docs(aio): update docs error in guide/http (#23567)

Updates documentation to include examples for both req.flush and
req.error in http testing examples.

PR Close #23567

aio/content/examples/http/src/testing/http-client.spec.ts

@@ -150,7 +150,7 @@ describe('HttpClient testing', () => {
 
     // Create mock ErrorEvent, raised when something goes wrong at the network level.
     // Connection timeout, DNS error, offline, etc
-    const errorEvent = new ErrorEvent('so sad', {
+    const mockError = new ErrorEvent('Network error', {
       message: emsg,
       // #enddocregion network-error
       // The rest of this is optional and not used.
@@ -162,7 +162,7 @@ describe('HttpClient testing', () => {
     });
 
     // Respond with mock error
-    req.error(errorEvent);
+    req.error(mockError);
   });
   // #enddocregion network-error
 

aio/content/guide/http.md

@@ -1030,10 +1030,18 @@ you are responsible for flushing and verifying them.
 
 You should test the app's defenses against HTTP requests that fail.
 
-Call `request.error()` with an `ErrorEvent` instead of `request.flush()`, as in this example.
+Call `request.flush()` with an error message, as seen in the following example.
 
 <code-example 
   path="http/src/testing/http-client.spec.ts"
   region="404" 
   linenums="false">
 </code-example>
+
+Alternatively, you can call `request.error()` with an `ErrorEvent`.
+
+<code-example
+  path="http/src/testing/http-client.spec.ts"
+  region="network-error"
+  linenums="false">
+</code-example>