MULE-8124: Adding body to listener not found response, functional test c...

...ase for missing listener scenario and unit test case for missing path scenario.
Asserting the status code of an HTTP response is definitely not an overkill, is a matcher that we could use broadly in all our tests. 
An other problem is that this callAndAssertStatus method is hiding the request that is sent to the server, making the test less readable.