Align Container tests with behavior from 37e642.

This changes the expected result in `cloudfiles_container_test#test_cdn_metadata_fails` to match the behavior introduced by 37e6425 (and renames it to match what it's actually testing). Fixes this test regression on `master`:

```
  1) Failure:
test_cdn_metadata_fails(CloudfilesContainerTest) [/home/ash/code/ruby-cloudfiles/test/cloudfiles_container_test.rb:448]:
CloudFiles::Exception::NoSuchContainer expected but nothing was raised.
```