Add Feature annotations to document tests

Also fix the assertNoCriticalErrors function, which is used by
the bigFile test.
The sensible behaviour would be to show an error message in the standard way informing the user that the specified file cannot be accessed, and that it may have been deleted or be on media that has been ejected.

As for what I expect Zanata to do, I'm not sure - it should at least indicate that the upload failed. It looks like this test is actually verifying that there is a message indicating successful upload, which doesn't seem very sensible.