Properly catch import errors in apt

When one accidentally tries to run this module as a user, he gets the error message that python-apt must be installed, no matter what. Because importing apt will trigger an exception as a regular user. Explicitly catching the ImportError will let the exception bubble. The exception clearly says Permission denied somewhere, and the user has a better idea, what he must fix.
Merge pull request #6480 from do3cc/patch-1

Properly catch import errors in apt