Added an integration-test that tests the exception-handling

Added an integration-test that tests the exception-handling

* The integration-test uses WebTestClient to test the exception-handling
* Since this is a real integration-test, the exception-handling behave
  exactly the same way
* The test-setup with the WebTestClient needs an additional dependency, to
  work correctly - without it, the WebTestClient-Bean would not be intanciated
* The added test ExceptionHandlingApplicationIT tests exaclty the same stuff,
  as ExceptionHandlingApplicationTest - this is no good test-design, but it
  serves the intention, to compare the two approaches
* Since Spring-Boot configures the surefire-maven-plugin to ignore all tests,
  that end with IT, the failsafe-maven-plugin has to be added to automatically
  fire the new test - remember: you have to call "mvn verify" to fire IT