Remove open rescue from Cli tests

This isn't actually as useful as I first planned. In most cases it obscures
the real exception when you're writing new tests and they fail. Because this
is a unit test and we're mocking the other classes that get called we'd
never expect to hit this. If we need to test the end-to-end behaviour it
would probably belong in an integration tests, rather than here.
Merge pull request #74 from alphagov/remove_rescue_from_cli_tests

Remove open rescue from Cli tests