Missing machine image results in silent error

In ~ubuntu-server-ec2-testing-dev/ubuntu-server-ec2-testing/ubuntu-server-ec2-testing/

execute_ubuntu_ec2_test.py calls retrieve_mi to find a machine image to run for the testcase. However if none is returned no error is logged and testing continues. Same for a failure to locate a testcase.

The user is specifying the search criteria for which relates to a particular image. If that image is missing it should be treated and reported as an error.