apt: fix for failure case

When failing to recognize the lsbdistid fact, the apt class would not
fail gracefully with our error message, but complaining that fail() is a
statement function, not an rvalue function (so it doesn't return a value).

This should fix that case and fail gracefully

Change-Id: I29a617352d5194f0e25c7f6df4e83a83d0dffb81
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>