CSPACE-4572: Services client tests more widely check response status codes prior...

CSPACE-4572: Services client tests more widely check response status codes prior to getting entity bodies from those responses, thus helping prevent actual error responses from being masked by 'Could not find JaxbContextFinder for media type: text/plain ...' messages. In nearly all such places prior to calling getEntity(), also replaced more verbose checking of status codes with calls to assertStatusCode() method in a base client test class.