Warn if configuration file not found, fail if missing auths or modules

[candlerb]

Fixes #1456

(Admittedly it's annoying to have to plumb through a logger just for one message)

[candlerb]

The test failure for ci/circleci: generator is unrelated to this change.

...
E: Failed to fetch http://archive.ubuntu.com/ubuntu/pool/universe/p/p7zip/p7zip_16.02%2bdfsg-8_amd64.deb  Unable to connect to archive.ubuntu.com:http: [IP: 185.125.190.83 80]
E: Failed to fetch http://archive.ubuntu.com/ubuntu/pool/universe/p/p7zip/p7zip-full_16.02%2bdfsg-8_amd64.deb  Unable to connect to archive.ubuntu.com:http: [IP: 185.125.190.83 80]
E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?