Improve e2e test to smoke test login screen

There were errors on the console about unfound backend paths. No the 
test fails when such errors appear.

e2e/src/app.e2e-spec.ts

+import { browser, logging } from 'protractor';
+
 import LoginPage from './app.po';
 
 describe('Testcenter Frontend', () => {
@@ -6,4 +8,12 @@ describe('Testcenter Frontend', () => {
     await expect(LoginPage.getFirstCardTitle()).toEqual('Anmelden');
     await expect(LoginPage.getSecondCardTitle()).toEqual('IQB-Testcenter');
   });
+
+  afterEach(async () => {
+    // Assert that there are no errors emitted from the browser
+    const logs = await browser.manage().logs().get(logging.Type.BROWSER);
+    expect(logs).not.toContain(jasmine.objectContaining({
+      level: logging.Level.SEVERE
+    } as logging.Entry));
+  });
 });